[imp] Horde_Prefs_Scope::serialize() must return a string or NULL

[Kein Name]

Hello List,

I am running the Horde Webmail Edition.
>From time to time I see this error, when certain users login:

Jun 21 09:48:33 webmailh5 HORDE: [imp] Login success for user at example.com
(192.168.1.1) to {imap://mail.example.com/} [pid 12661 on line 157 of
"/var/www/webmail/imp/lib/Auth.php"]
Jun 21 09:48:41 webmailh5 HORDE: [turba] Horde_Prefs_Scope::serialize()
must return a string or NULL [pid 12627 on line 62 of
"/usr/share/php/Horde/Prefs/Cache/HordeCache.php"]
Jun 21 09:48:41 webmailh5 HORDE: [imp] Horde_Prefs_Scope::serialize() must
return a string or NULL [pid 12627 on line 1679 of
"/usr/share/php/Horde/Registry.php"]

Jun 21 09:48:41 webmailh5 HORDE: [horde] Mail is not activated. [pid 12627
on line 1532 of "/usr/share/php/Horde/Registry.php"]

I looked into the horde_prefs table and the settings for this user, but
could not find any special characters etc.

I found this info:
http://horde.690.n7.nabble.com/usr-bin-kronolith-agenda-
Horde-Prefs-Scope-serialize-must-return-a-string-or-NULL-
td134336.html#a134347

However the Scope.php does not exist anymore in the most recent version.

How can I debug this matter further?

Thanks!
Stefan