[horde] hordeauth via imap works, but not for ActiveSync

[Dr. Andreas Kluge]

I have the following problem:

Horde Webmail 5.1.3 is up and running. Horde authentication is handled
by imp and works well. ActiveSync from iPad (IOS 7.03) with
Exchange-driver shows the error "Accountinformationen konnten nicht
überprüft werden" (Account-Informations couldn't be checked) on the
iPad.
In hordelog the error-information looks like:
###
...
Nov 26 20:23:23 webmail5 HORDE: [horde] User kluge [139.20.208.21]
logged out of Horde [pid 49542 on line 117 of
"/usr/local/www/horde/login.php"]
Nov 26 20:27:08 webmail5 HORDE: [horde] Login success for kluge
[139.20.208.21] to horde. [pid 63472 on line 165 of
"/usr/local/www/horde/login.php"]
Nov 26 20:27:09 webmail5 HORDE: [imp] Login success for kluge (Horde
user kluge) [139.20.208.21] to {mailserver.tu-freiberg.de:11143
[imap]} [pid 63472 on line 157 of
"/usr/local/www/horde/imp/lib/Auth.php"]
Nov 26 20:55:32 webmail5 HORDE: [horde] Verbindung zum E-Mail-Server
fehlgeschlagen. [pid 64234 on line 62 of
"/usr/local/share/pear/Horde/Core/ActiveSync/Auth.php"]
Nov 26 20:55:32 webmail5 HORDE: [horde] Login failed from ActiveSync
client for user kluge. [pid 64234 on line 558 of
"/usr/local/share/pear/Horde/ActiveSync.php"]
Nov 26 20:55:32 webmail5 HORDE: [horde] Verbindung zum E-Mail-Server
fehlgeschlagen. [pid 64234 on line 62 of
"/usr/local/share/pear/Horde/Core/ActiveSync/Auth.php"]
Nov 26 20:55:32 webmail5 HORDE: [horde] Login failed from ActiveSync
client for user kluge. [pid 64234 on line 558 of
"/usr/local/share/pear/Horde/ActiveSync.php"]

...
###
The sucessful login with hordeauth is logged in mailservers maillog,
but for the attempt with ActiveSync there is no message.
Tests with tcpdump on the webmail-server shows that ther is no
communication between (Horde-) webmailserver and (Dovecot-) mailserver
while the attempt to connect with iPad to webmailserver.
ActiveSync with the same iPad and the same configuration works well
with Horde Webmail 5.0.4.
Therefore it seems to me like a bug.

Any help is appreciated.

Best Regards
Andreas