Correct

I'm assuming that hasn't happened again/is not reproducible?

The 8th element of the array contains an 512,648,440 element array:

i:8;a:512648440:{ [...]

I have no idea how that could happen.  You need to look at either UID 
103738 or 103731 in INBOX.horde.cvs to figure out why that is happening.

Emptying cache or logging out doesn't help either.

[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP Fatal 
error:  Allowed memory size of 268435456 bytes exhausted (tried to 
allocate 4294967296 bytes) in 
/home/jan/horde-develop/framework/Mime/lib/Horde/Mime/Part.php on line 
2275, referer: http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP Stack 
trace:, referer: http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   1. 
{main}() /home/jan/horde-develop/horde/services/ajax.php:0, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   2. 
Horde_Core_Ajax_Application->doAction() 
/home/jan/horde-develop/horde/services/ajax.php:56, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   3. 
call_user_func(array (0 => class IMP_Ajax_Application_Handler_Common { 
protected $_base = class IMP_Ajax_Application { public $mbox = class 
IMP_Mailbox { ... }; public $queue = class IMP_Ajax_Queue { ... }; 
public $data = NULL; public $tasks = NULL; protected $_action = 
'showMessage'; protected $_app = 'imp'; protected $_handlers = array 
(...); protected $_vars = class Horde_Variables { ... } }; protected 
$_external = array (); protected $_ignored = array (); protected 
$_readOnly = array () }, 1 => 'showMessage')) 
/home/jan/horde-develop/framework/Core/lib/Horde/Core/Ajax/Application.php:155, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   4. 
IMP_Ajax_Application_Handler_Common->showMessage() 
/home/jan/horde-develop/framework/Core/lib/Horde/Core/Ajax/Application.php:155, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   5. 
IMP_Ajax_Application->viewPortData($change = TRUE) 
/home/jan/horde-develop/imp/lib/Ajax/Application/Handler/Common.php:680, 
referer: http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   6. 
IMP_Ajax_Application_ListMessages->listMessages($args = array 
('change' => TRUE, 'mbox' => 'INBOX.horde.cvs', 'applyfilter' => NULL, 
'cache' => 
'{20}SU5CT1guaG9yZGUuY3Zz70496,96466,98087,98466,98725,98727:98728,98852,99533,100174,100395,101508,103521,103729:103739', 'cacheid' => 'V991562206|H10668|3|1|D022513', 'delhide' => NULL, 'initial' => NULL, 'qsearch' => NULL, 'qsearchfield' => NULL, 'qsearchfilter' => NULL, 'qsearchflag' => NULL, 'qsearchflagnot' => NULL, 'qsearchmbox' => NULL, 'rangeslice' => NULL, 'sortby' => NULL, 'sortdir' => NULL, 'slice_start' => 1, 'slice_end' => 154)) /home/jan/horde-develop/imp/lib/Ajax/Application.php:241, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   7. 
IMP_Ajax_Application_ListMessages->_getOverviewData($mbox = class 
IMP_Mailbox { protected $_cache = array ('n' => array (0 => '.', 2 => 
'INBOX.', 4 => 1), 'd' => 'horde.cvs', 'l' => TRUE, 'ro' => FALSE, 'a' 
=> 'lrswipkxtea', 'v' => '991562206'); protected $_changed = 0; 
protected $_import = NULL; protected $_mbox = 'INBOX.horde.cvs' }, 
$msglist = array (0 => 3, 1 => 9)) 
/home/jan/horde-develop/imp/lib/Ajax/Application/ListMessages.php:395, 
referer: http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   8. 
IMP_Mailbox_List->getMailboxArray($msgnum = array (0 => 3, 1 => 9), 
$options = array ('headers' => TRUE, 'type' => '1')) 
/home/jan/horde-develop/imp/lib/Ajax/Application/ListMessages.php:435, 
referer: http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP   9. 
IMP_Imap->fetch('INBOX.horde.cvs', class Horde_Imap_Client_Fetch_Query 
{ protected $_data = array (9 => TRUE, 10 => TRUE, 12 => TRUE, 13 => 
TRUE, 8 => array ('imp' => array (...))) }, array ('ids' => class 
Horde_Imap_Client_Ids { public $duplicates = FALSE; protected $_ids = 
array (0 => 103738, 1 => 103731); protected $_sequence = FALSE; 
protected $_sorted = FALSE })) 
/home/jan/horde-develop/imp/lib/Mailbox/List.php:180, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP  10. 
IMP_Imap->__call($method = 'fetch', $params = array (0 => 
'INBOX.horde.cvs', 1 => class Horde_Imap_Client_Fetch_Query { 
protected $_data = array (9 => TRUE, 10 => TRUE, 12 => TRUE, 13 => 
TRUE, 8 => array (...)) }, 2 => array ('ids' => class 
Horde_Imap_Client_Ids { public $duplicates = FALSE; protected $_ids = 
array (...); protected $_sequence = FALSE; protected $_sorted = FALSE 
}))) /home/jan/horde-develop/imp/lib/Mailbox/List.php:180, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP  11. 
call_user_func_array(array (0 => class Horde_Imap_Client_Socket { 
protected $_fetch = class Horde_Imap_Client_Fetch_Results { protected 
$_data = array (...); protected $_keyType = 1; protected $_obClass = 
'Horde_Imap_Client_Data_Fetch' }; protected $_statusFields = array 
('messages' => 1, 'recent' => 2, 'uidnext' => 4, 'uidvalidity' => 8, 
'unseen' => 16, 'firstunseen' => 64, 'flags' => 128, 'permflags' => 
256, 'uidnotsticky' => 4096, 'highestmodseq' => 512); protected 
$_stream = resource(19) of type (stream); protected $_tag = 10; public 
$cacheFields = array (9 => 'HICenv', 10 => 'HICflags', 8 => 'HIChdrs', 
11 => 'HICdate', 12 => 'HICsize', 1 => 'HICstruct'); public $changed = 
TRUE; protected $_cache = class Horde_Imap_Client_Cache { protected 
$_base = ...; protected $_cache = class Horde_Cache { ... }; protected 
$_data = array (...); protected $_debug = class 
Horde_Imap_Client_Base_Debug { ... }; protected $_loaded = array 
(...); protected $_params = array (...); protected $_slicemap = array 
(...); protected $_update = array (...) }; protected $_debug = class 
Horde_Imap_Client_Base_Debug { public $debug = TRUE; protected 
$_buffer = NULL; protected $_stream = resource(18) of type (stream); 
protected $_time = 1361787821.8279 }; protected $_fetchDataClass = 
'Horde_Imap_Client_Data_Fetch'; protected $_init = array ('enabled' => 
array (...), 'namespace' => array (...), 's_charset' => array (...), 
'imapproxy' => TRUE, 'authmethod' => 'LOGIN', 'capability' => array 
(...)); protected $_isAuthenticated = TRUE; protected $_isSecure = 
FALSE; protected $_mode = 2; protected $_params = array ('encryptKey' 
=> array (...), 'hostspec' => 'localhost', 'secure' => FALSE, 
'timeout' => 30, 'envelope_addrs' => 1000, 'envelope_string' => 2048, 
'capability_ignore' => array (...), 'debug' => '/tmp/imap.log', 
'password' => 'A\\027gr\xdf\xb6\xe0\x8a\xfa\\f\xf9\xdfd\x84\x8c\xd7', 
'port' => 1143, 'username' => 'jan', 'cache' => array (...), 
'_passencrypt' => TRUE); protected $_selected = class 
Horde_Imap_Client_Mailbox { protected $_utf7imap = TRUE; protected 
$_utf8 = 'INBOX.horde.cvs' }; protected $_temp = array ('loginerr' => 
class Horde_Imap_Client_Exception { ... }, 'referral' => NULL, 
'lastcmd' => class Horde_Imap_Client_Interaction_Client { ... }, 
'modseqs' => array (...), 'modseqs_nouid' => array (...), 'proxyreuse' 
=> TRUE, 'mailbox_ob' => array (...), 'fetchcmd' => array (...), 
'headers_caching' => array (...)) }, 1 => 'fetch'), array (0 => class 
Horde_Imap_Client_Mailbox { protected $_utf7imap = NULL; protected 
$_utf8 = 'INBOX.horde.cvs' }, 1 => class Horde_Imap_Client_Fetch_Query 
{ protected $_data = array (9 => TRUE, 10 => TRUE, 12 => TRUE, 13 => 
TRUE, 8 => array (...)) }, 2 => array ('ids' => class 
Horde_Imap_Client_Ids { public $duplicates = FALSE; protected $_ids = 
array (...); protected $_sequence = FALSE; protected $_sorted = FALSE 
}))) /home/jan/horde-develop/imp/lib/Imap.php:388, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP  12. 
Horde_Imap_Client_Base->fetch($mailbox = class 
Horde_Imap_Client_Mailbox { protected $_utf7imap = NULL; protected 
$_utf8 = 'INBOX.horde.cvs' }, $query = class 
Horde_Imap_Client_Fetch_Query { protected $_data = array (9 => TRUE, 
10 => TRUE, 12 => TRUE, 13 => TRUE, 8 => array ('imp' => array (...))) 
}, $options = array ('ids' => class Horde_Imap_Client_Ids { public 
$duplicates = FALSE; protected $_ids = array (0 => 103738, 1 => 
103731); protected $_sequence = FALSE; protected $_sorted = FALSE })) 
/home/jan/horde-develop/imp/lib/Imap.php:388, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP  13. 
Horde_Imap_Client_Cache->get($mailbox = class 
Horde_Imap_Client_Mailbox { protected $_utf7imap = TRUE; protected 
$_utf8 = 'INBOX.horde.cvs' }, $uids = array (0 => 103738, 1 => 
103731), $fields = array (0 => 'HICenv', 1 => 'HICflags', 2 => 
'HIChdrs', 3 => 'HICsize'), $uidvalid = '991562206') 
/home/jan/horde-develop/framework/Imap_Client/lib/Horde/Imap/Client/Base.php:2549, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP  14. 
unserialize('a:5:{s:6:"HICenv";C:31:"Horde_Imap_Client_Data_Envelope":750:{a:2:{s:1:"d";C:18:"Horde_Mime_Headers":692:{a:3:{i:0;i:2;i:1;a:7:{s:4:"date";a:2:{s:1:"h";s:4:"date";s:1:"v";s:29:"Mon, 25 Feb 2013 00:33:33 GMT";}s:7:"subject";a:2:{s:1:"h";s:7:"subject";s:1:"v";s:80:"[commits] Horde branch imp_6_1 updated.\\td6b64567a2a42f722b042bd63024919f8f7c95ec";}s:4:"from";a:2:{s:1:"h";s:4:"from";s:1:"v";s:37:"Michael M Slusarz <slusarz@horde.org>";}s:6:"sender";a:2:{s:1:"h";s:6:"sender";s:1:"v";s:31:"commits-bounces@lists....') /home/jan/horde-develop/framework/Imap_Client/lib/Horde/Imap/Client/Cache.php:267, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP  15. 
Horde_Mime_Part->unserialize($data = 
'a:20:{i:0;i:1;i:1;s:4:"text";i:2;s:5:"plain";i:3;s:4:"7bit";i:4;a:0:{}i:5;s:0:"";i:6;s:0:"";i:7;a:1:{s:4:"size";s:4:"1231";}i:8;a:512648440:{s:7:"charset";s:8:"us-ascii";}i:9;a:0:{}i:10;s:1:"1";i:11;s:1:"\\n";i:12;a:0:{}i:13;N;i:14;i:1231;i:15;N;i:16;N;i:17;b:0;i:18;b:0;i:19;N;}') /home/jan/horde-develop/framework/Imap_Client/lib/Horde/Imap/Client/Cache.php:267, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox
[Mon Feb 25 11:23:41 2013] [error] [client 192.168.60.117] PHP  16. 
unserialize('a:20:{i:0;i:1;i:1;s:4:"text";i:2;s:5:"plain";i:3;s:4:"7bit";i:4;a:0:{}i:5;s:0:"";i:6;s:0:"";i:7;a:1:{s:4:"size";s:4:"1231";}i:8;a:512648440:{s:7:"charset";s:8:"us-ascii";}i:9;a:0:{}i:10;s:1:"1";i:11;s:1:"\\n";i:12;a:0:{}i:13;N;i:14;i:1231;i:15;N;i:16;N;i:17;b:0;i:18;b:0;i:19;N;}') /home/jan/horde-develop/framework/Mime/lib/Horde/Mime/Part.php:2275, referer: 
http://neo.wg.de/headhorde/imp/dynamic.php?page=mailbox