Well possible. I haven't seen this even before that commit after I 
cleared the cache, so it was probably a side-effect of some polluted 
cache. I'll report back if this ever happens again.

Was this fixed by:

commit fdbed86d99588110a1bea2f3497948a45378d990
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Feb 28 21:42:48 2012 -0700

     Bug #11026: Better fix to invalidate cache entries.

(The line numbers don't line up anymore.)

This happens when opening some mailboxes. If I open the same mailbox 
again, the message list is loaded, but no longer in the original sort 
order (by thread) but in arrival order.

Fatal error: Unsupported operand types in 
/home/jan/horde-git/framework/Imap_Client/lib/Horde/Imap/Client/Cache.php on 
line 539
Call Stack
#        Time        Memory        Function        Location
1        0,0001        652288        {main}( )        ../ajax.php:0
2        0,5281        13260152        IMP_Ajax_Application->doAction( )        ../ajax.php:60
3        0,5281        13260152        Horde_Core_Ajax_Application->doAction( 
)        ../Application.php:95
4        0,5281        13260784        call_user_func ( array (0 => class 
IMP_Ajax_Application { public $notify = TRUE; protected $_mbox = class 
IMP_Mailbox { public $cache = array (...); public $changed = 1; 
protected $_import = NULL; protected $_mbox = 
'INBOX.Mailinglisten.Cyrus' }; protected $_queue = class 
IMP_Ajax_Queue { protected $_flag = array (...); protected $_poll = 
array (...); protected $_quota = FALSE }; protected $_suppress = NULL; 
protected $_readOnly = array (0 => 'html2Text', 1 => 'text2Html'); 
protected $_app = 'imp'; protected $_action = 'viewPort'; protected 
$_vars = class Horde_Variables { protected $_expected = array (...); 
protected $_sanitized = FALSE; protected $_vars = array (...) }; 
protected $_defaultDomain = NULL }, 1 => 'viewPort') 
)        ../Application.php:95
5        0,5281        13260784        IMP_Ajax_Application->viewPort( )        ../Application.php:95
6        0,6565        14813496        IMP_Ajax_Application->_viewPortData( TRUE, ??? 
)        ../Application.php:724
7        0,6572        14831456        IMP_Views_ListMessages->listMessages( array 
('change' => TRUE, 'mbox' => 'INBOX.Mailinglisten.Cyrus', 
'applyfilter' => NULL, 'cache' => NULL, 'cacheid' => NULL, 'delhide' 
=> NULL, 'initial' => '1', 'qsearch' => NULL, 'qsearchfield' => NULL, 
'qsearchfilter' => NULL, 'qsearchflag' => NULL, 'qsearchflagnot' => 
NULL, 'qsearchmbox' => NULL, 'rangeslice' => NULL, 'requestid' => 
'11', 'sortby' => NULL, 'sortdir' => NULL, 'after' => 66, 'before' => 
44, 'slice_start' => 1, 'slice_end' => 111) )        ../Application.php:1992
8        0,8380        15460896        IMP_Views_ListMessages->_getOverviewData( class 
IMP_Mailbox { public $cache = array ('n' => array (0 => '.', 2 => 
'INBOX.', 4 => 1), 'd' => 'Mailinglisten.Cyrus', 'ro' => FALSE, 'a' => 
'lrswipakxte'); public $changed = 1; protected $_import = NULL; 
protected $_mbox = 'INBOX.Mailinglisten.Cyrus' }, array (0 => 1, 1 => 
2, 2 => 3, 3 => 4, 4 => 5, 5 => 6) )        ../ListMessages.php:391
9        0,8382        15461992        IMP_Mailbox_List->getMailboxArray( array (0 => 1, 1 
=> 2, 2 => 3, 3 => 4, 4 => 5, 5 => 6), array ('headers' => TRUE, 
'type' => '1') )        ../ListMessages.php:425
10        0,8393        15510848        IMP_Imap->fetch( 'INBOX.Mailinglisten.Cyrus', 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 { protected $_ids = 
array (0 => 17807, 1 => 17808, 2 => 17811, 3 => 17812, 4 => 17809, 5 
=> 17810); protected $_sequence = FALSE; protected $_utilsClass = 
'Horde_Imap_Client_Utils' }) )        ../List.php:170
11        0,8393        15511600        IMP_Imap->__call( 'fetch', array (0 => 
'INBOX.Mailinglisten.Cyrus', 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 { protected $_ids = array (...); protected 
$_sequence = FALSE; protected $_utilsClass = 'Horde_Imap_Client_Utils' 
})) )        ../List.php:170
12        0,8393        15512232        call_user_func_array ( array (0 => class 
Horde_Imap_Client_Socket { protected $_tag = 5; protected $_stream = 
resource(87) of type (stream); public $cache = class 
Horde_Imap_Client_Cache { protected $_base = ...; protected $_cache = 
class Horde_Cache { ... }; protected $_data = array (...); protected 
$_loaded = array (...); protected $_params = array (...); protected 
$_slicemap = array (...); protected $_save = array (...) }; public 
$cacheFields = array (9 => 'HICenv', 10 => 'HICflags', 8 => 'HIChdrs', 
11 => 'HICdate', 12 => 'HICsize', 1 => 'HICstruct'); public $changed = 
TRUE; protected $_debug = NULL; 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', 'log' => NULL, 'port' => 
1143, 'secure' => FALSE, 'timeout' => 30, 'password' => 
'þ›\021³Ú±\v}', 'statuscache' => TRUE, 'username' => 'jan', 'cache' => 
array (...), '_passencrypt' => TRUE); protected $_selected = class 
Horde_Imap_Client_Mailbox { protected $_utf7imap = TRUE; protected 
$_utf8 = 'INBOX.Mailinglisten.Cyrus' }; protected $_temp = array 
('no_cap' => TRUE, 'loginerr' => 'LOGIN_AUTHENTICATIONFAILED', 
'loginerrmsg' => 'Der E-Mail-Server hat Authentifizierung 
zurückgewiesen.', 'referral' => NULL, 'fr_ob' => class stdClass { ... 
}, 'fetchresp' => class stdClass { ... }, 'parsestatuserr' => NULL, 
'proxyreuse' => TRUE, 'mailbox' => array (...), 'literal8' => array 
(...), 'threadparse' => array (...), 'myrights' => class 
Horde_Imap_Client_Data_Acl { ... }, 'headers_caching' => array (...)); 
protected $_utils = class Horde_Imap_Client_Utils { }; protected 
$_utilsClass = 'Horde_Imap_Client_Utils' }, 1 => 'fetch'), array (0 => 
'INBOX.Mailinglisten.Cyrus', 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 { protected $_ids = array (...); protected 
$_sequence = FALSE; protected $_utilsClass = 'Horde_Imap_Client_Utils' 
})) )        ../Imap.php:343
13        0,8393        15512976        Horde_Imap_Client_Base->fetch( 
'INBOX.Mailinglisten.Cyrus', 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 { protected $_ids = array (0 => 17807, 1 => 
17808, 2 => 17811, 3 => 17812, 4 => 17809, 5 => 17810); protected 
$_sequence = FALSE; protected $_utilsClass = 'Horde_Imap_Client_Utils' 
}) )        ../Imap.php:343
14        0,8411        15547304        Horde_Imap_Client_Cache->get( class 
Horde_Imap_Client_Mailbox { protected $_utf7imap = TRUE; protected 
$_utf8 = 'INBOX.Mailinglisten.Cyrus' }, array (0 => 17807, 1 => 17808, 
2 => 17811, 3 => 17812, 4 => 17809, 5 => 17810), array (0 => 'HICenv', 
1 => 'HIChdrs', 2 => 'HICsize'), '1118412327' )        ../Base.php:2425
15        0,8412        15547776        Horde_Imap_Client_Cache->_loadUIDs( 
'INBOX.Mailinglisten.Cyrus', array (0 => 17807, 1 => 17808, 2 => 
17811, 3 => 17812, 4 => 17809, 5 => 17810), '1118412327' 
)        ../Cache.php:268
16        0,8412        15547776        Horde_Imap_Client_Cache->_loadMailbox( 
'INBOX.Mailinglisten.Cyrus', array (0 => 17807, 1 => 17808, 2 => 
17811, 3 => 17812, 4 => 17809, 5 => 17810), '1118412327' 
)        ../Cache.php:592
17        0,8434        15588352        Horde_Imap_Client_Cache->_loadSlice( 
'INBOX.Mailinglisten.Cyrus', 0 )        ../Cache.php:482