This commit solved the issue for me too.
Thanks!

Anyway, regardind this error I've found my log full of this messages:

Jul 12 22:10:36 isengard HORDE: [horde] PHP ERROR: openssl_encrypt(): 
Using an empty Initialization Vector (iv) is potentially insecure and 
not recommended [pid 20393 on line 44 of 
"/usr/share/php/Horde/Crypt/Blowfish/Openssl.php"]
Jul 12 22:10:37 isengard HORDE: [kronolith] PHP ERROR: 
openssl_encrypt(): Using an empty Initialization Vector (iv) is 
potentially insecure and not recommended [pid 20393 on line 44 of 
"/usr/share/php/Horde/Crypt/Blowfish/Openssl.php"]
Jul 12 22:10:37 isengard HORDE: [imp] PHP ERROR: openssl_encrypt(): 
Using an empty Initialization Vector (iv) is potentially insecure and 
not recommended [pid 20393 on line 44 of 
"/usr/share/php/Horde/Crypt/Blowfish/Openssl.php"]
Jul 12 22:10:39 isengard HORDE: [gollem] PHP ERROR: openssl_encrypt(): 
Using an empty Initialization Vector (iv) is potentially insecure and 
not recommended [pid 20393 on line 44 of 
"/usr/share/php/Horde/Crypt/Blowfish/Openssl.php"]

Well, I was testing an unofficial PHP version on that server 
(5.4.17RC1 from a PPA).
I reverted to official 5.3.2 and the problem seems solved.

Wow, pleasantly surprised :)

This fixed the problem on both of my two devices. Thanks!

In the sync.log that was sent to me privately, I discovered a "off by 
one error" where we were sending one more entry to the client than was 
requested (by the WINDOWSIZE element). This shouldn't cause a problem 
since the WINDOWSIZE limit is said to be a "SHOULD", not a "MUST". 
However, with all thing ActiveSync, you never know what the client is 
going to do....

Can anyone verify this does or does not fix this for them?

In your horde.log you have:

Jul 12 11:57:33 isengard HORDE:  1. 
Horde_Rpc_ActiveSync->getResponse() 
/srv/www/applications/horde/rpc.php:156  2. 
Horde_ActiveSync->handleRequest() 
/usr/share/php/Horde/Rpc/ActiveSync.php:141  3. 
Horde_ActiveSync->authenticate() 
/usr/share/php/Horde/ActiveSync.php:736  4. 
Horde_Core_ActiveSync_Driver->authenticate() 
/usr/share/php/Horde/ActiveSync.php:561  5. 
Horde_Core_Auth_Application->authenticate() 
/usr/share/php/Horde/Core/ActiveSync/Driver.php:181  6. 
Horde_Core_Auth_Application->_setAuth() 
/usr/share/php/Horde/Core/Auth/Application.php:138  7. 
Horde_Registry->setAuth() 
/usr/share/php/Horde/Core/Auth/Application.php:639  8. 
Horde_Registry->setAuthCredential() 
/usr/share/php/Horde/Registry.php:2489  9. Horde_Secret->write() 
/usr/share/php/Horde/Registry.php:2381 10. 
Horde_Crypt_Blowfish->encrypt() /usr/share/php/Horde/Secret.php:75 11. 
Horde_Crypt_Blowfish_Openssl->encrypt() 
/usr/share/php/Horde/Crypt/Blowfish.php:110 12. openssl_encrypt() 
/usr/share/php/Horde/Crypt/Blowfish/Openssl.php:44 13. Horde_Error


No idea where this is coming from, do you?

Changes have been made in Git (master):

commit 42dc55cded76aa30012506c924fc9af589e1bb52
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Fri Jul 12 12:30:13 2013 -0400

     Fix off by one error.

     Very slight chance this might be related to Bug: 12443. The WINDOWSIZE
     limit is a SHOULD, not a MUST so clients are supposed to be able to
     deal with receiving more than WINDOWSIZE entries, but you never know...

  .../lib/Horde/ActiveSync/Request/Sync.php          |    8 ++++----
  1 files changed, 4 insertions(+), 4 deletions(-)

http://git.horde.org/horde-git/-/commit/42dc55cded76aa30012506c924fc9af589e1bb52

Hi, I have the same problem with my 2 servers.
Since one of the last ActiveSync Upgrades contacts won't sync anymore.

I've found a strange behaviour: if I have an empty address book, and 
manually add some contacts, my phone will sync.

If I have an already populated address book , with (for example) 100 
contacts, it won't sync.
Also, if I import a batch of contact in my already syncronized 
addressbook, my phone won't see them.

To make a test , I've downgraded Horde_Core to 2.6.0 and 
Horde_ActiveSync to 2.5.1, and the problem is solved.

I attach my log.

This, in itself, is not an error. It's a NOTICE indicating that for 
some reason the client has reissued a SYNC request using the same 
synckey more than once.

You need to find out why the client is not accepting the server 
response. I can verify that this works fine on many different clients 
(including multiple versions of Android). If you provide the FULL 
synclog (you can email it to me directly), I can try to look for you 
as well.

After Upgrading to Horde 5.1.1 syncing contacts fails with the 
following error:

2013-07-10T10:02:45+02:00 NOTICE: [26689] Error saving state for 
synckey {51dd11a6-1b10-4f11-85f5-773b2e04c152}2: SQLSTATE[23000]: 
Integrity constraint violation: 1062 Duplicate entry 
'{51dd11a6-1b10-4f11-85f5-773b2e04c152}2' for key 'PRIMARY' - removing 
previous sync state and trying again.

I deleted the contacts that I saw in the log file immediately before 
this error occurred, and also the following contact.

I can reproduce this problem on two different devices, both running 
Android 4.2.2, with two different Horde accounts.

Syncing calendars works without any problems.