No, I didn't figure out that it was necessary. However, I've now 
re-tested and the error message is really gone.

Thanks!

Did you recreate the account on the phone?

Might be. Didn't get fixed though with the latest commits. I've now 
updated to 75511fd7e5a80b85efaf19bef67d992b004df4a1 and the bug still 
persists.

Probably duplicate of Bug: 12239

Ok, thanks for the pointers. Here is some more info & backtrace:

2013-05-05T11:39:18+03:00 DEBUG: Variable information:
array(19) {
   [0]=>
   array(2) {
     ["read"]=>
     int(0)
     ["flagged"]=>
     int(0)
   }
   [1]=>
   int(43400)
   [2]=>
   int(43405)
   [3]=>
   int(43421)
   [4]=>
   int(43422)
   [5]=>
   int(43423)
   [6]=>
   int(43424)
   [7]=>
   int(43425)
   [8]=>
   int(43426)
   [9]=>
   int(43427)
   [10]=>
   int(43429)
   [11]=>
   int(43431)
   [12]=>
   int(43432)
   [13]=>
   int(43433)
   [14]=>
   int(43440)
   [15]=>
   int(43441)
   [16]=>
   int(43442)
   [17]=>
   int(43443)
   [18]=>
   int(43444)
}

Backtrace:
  1. Horde_Rpc_ActiveSync->getResponse() horde/rpc.php:156
  2. Horde_ActiveSync->handleRequest() 
framework/Rpc/lib/Horde/Rpc/ActiveSync.php:141
  3. Horde_ActiveSync_Request_Base->handle() 
framework/ActiveSync/lib/Horde/ActiveSync.php:848
  4. Horde_ActiveSync_Request_Sync->_handle() 
framework/ActiveSync/lib/Horde/ActiveSync/Request/Base.php:236
  5. Horde_ActiveSync_Sync->init() 
framework/ActiveSync/lib/Horde/ActiveSync/Request/Sync.php:375
  6. Horde_ActiveSync_State_Sql->getChanges() 
framework/ActiveSync/lib/Horde/ActiveSync/Sync.php:149
  7. Horde_Core_ActiveSync_Driver->getServerChanges() 
framework/ActiveSync/lib/Horde/ActiveSync/State/Sql.php:825
  8. Horde_ActiveSync_Imap_Adapter->getMessageChanges() 
framework/Core/lib/Horde/Core/ActiveSync/Driver.php:648
  9. Horde_Imap_Client_Ids->__construct() 
framework/ActiveSync/lib/Horde/ActiveSync/Imap/Adapter.php:328
10. Horde_Imap_Client_Ids->add() 
framework/Imap_Client/lib/Horde/Imap/Client/Ids.php:82
11. Horde::debug() framework/Imap_Client/lib/Horde/Imap/Client/Ids.php:180

Yeah, EAS version changes do not seem to be picked up fully by clients 
automatically. They sometimes require a reboot, or in some cases, 
re-creating the account.

BTW, you can simply use print_r($this->_ids, true) to get a printable 
copy of the array values - or better yet, use 
Horde::debug($this->_ids); and this will output to a file named 
horde_debut.txt in your server's tmp directory.

This doesn't explain why the flip doesn't work. They are both arrays. 
The error complains that the value is NOT an array. We need both the 
value of the ids array as well as the backtrace when it doesn't equal 
an array. You can get a backtrace via either debug_bactrace() or just 
use Horde::debug which automatically generates it.

Ok, no wonder the log didn't show anything. There's something fishy 
going on as after I changed the activesync version to v14.1 and 
_rebooted_ the windows phone, it suddenly started to work just fine.
Now all the message bodies are being shown just fine (new & all, 
doesn't matter).

Ok, this seems to describe why the flip doesn't work ---> on windows 
client device
the _ids is an array as show below:

windows phone 8:

[03-May-2013 05:57:36 UTC] ERROR: _ids=[Array]
[03-May-2013 05:57:36 UTC] ERROR: array=[1]
[03-May-2013 05:57:36 UTC] ERROR: array=[2]
[03-May-2013 05:57:36 UTC] ERROR: array=[3]
[03-May-2013 05:57:36 UTC] ERROR: array=[4]
[03-May-2013 05:57:36 UTC] ERROR: array=[5]
[03-May-2013 05:57:36 UTC] ERROR: array=[6]
[03-May-2013 05:57:36 UTC] ERROR: array=[7]
[03-May-2013 05:57:36 UTC] ERROR: array=[8]
[03-May-2013 05:57:36 UTC] ERROR: array=[9]
[03-May-2013 05:57:36 UTC] ERROR: array=[10]
[03-May-2013 05:57:36 UTC] ERROR: array=[11]
[03-May-2013 05:57:36 UTC] ERROR: array=[12]
[03-May-2013 05:57:36 UTC] ERROR: array=[13]
[03-May-2013 05:57:36 UTC] ERROR: array=[14]
[03-May-2013 05:57:36 UTC] ERROR: array=[15]
[03-May-2013 05:57:36 UTC] ERROR: array=[16]
[03-May-2013 05:57:36 UTC] ERROR: array=[17]
[03-May-2013 05:57:36 UTC] ERROR: array=[18]
[03-May-2013 05:57:36 UTC] ERROR: array=[19]
[03-May-2013 05:57:36 UTC] ERROR: array=[20]

An android device:

[03-May-2013 05:57:36 UTC] ERROR: _ids=[Array]
[03-May-2013 05:57:36 UTC] ERROR: array=[386]

The output is retrieved using the following lines:

                     $errmsg=$this->_ids;
                     error_log("ERROR: _ids=[" . $errmsg . "]", 0);
                     foreach($this->_ids as $foobar) {
                         error_log("ERROR: array=[" . $foobar . "]", 0);
                     }

There was another report, Bug: 12203 from a Windows 8 device. The 
provided synclog didn't show anything wrong though. If you can provide 
a synclog please attach it, and reopen, Bug: 12203.

Thanks.

Can you provide a stack trace and/or the value of $this->_ids when you 
get this error?

It seems android devices (nexus 7 and galaxy s3) are working just fine 
with the latest activesync (v14.1), while my wifes nokia lumia 820 
generates the following error while trying to synchronize mails:
Used git revision: a9450a21b521361da536f15efcd4d96cd519bc9f

PHP ERROR: array_flip(): Can only flip STRING and INTEGER values! [ on 
line 182 of "/framework/Imap_Client/lib/Horde/Imap/Client/Ids.php"

Related or unrelated, the windows phone is able to list/show emails, 
but the email body is always completely blank.