[horde] Kronolith 4.2.27 on Horde 5.2.22 - No supported IMAP authentication method could be found.

[director_de at yahoo.de]

Using Horde 5.2.22, with kronolith 4.2.27 and trying to connect to use IMAP/Kolab as Storage backend, I get this error: 
Es wurde keine unterstützte Authentifizierungsmethode für IMAP gefunden.( No supported IMAP authentication method could be found. ) How can this being debugged, or resolved? In Log we find:
Dec 27 09:58:47 vm1024 HORDE[17568]: [kronolith] Horde_Prefs: Storing preference value (display_external_cals) [pid 17568 on line 532 of "/usr/share/horde/kronolith/lib/CalendarsManager.php"]Dec 27 09:58:47 vm1024 HORDE[17568]: [kronolith] Driver "Horde_Kolab_Storage_Driver_Imap": Retrieving namespaces. [pid 17568 on line 308 of "/usr/share/pear/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
How can this being debugged ? Which might went wrong?
Thanks for your help in advance!T