[horde] trouble with prefs

[A. Schulze]

Hello,

I've to migrate an old horde3 instance (yes, shame :-/)  user_prefs  
are stored in an LDAP-Server.
A new Horde5 instance operate with these prefs. That works well for new users.
But existing users get this error:

Horde_Prefs_Scope::serialize() must return a string or NULL
in /usr/share/php/Horde/Registry.php:1679
  1. Horde_Registry::appInit() /usr/share/horde/imp/index.php:19
  2. Horde_Registry->pushApp() /usr/share/php/Horde/Registry.php:298
  3. Horde_Registry->_pushAppError() /usr/share/php/Horde/Registry.php:1622
  4. Horde_Registry::appInit() /usr/share/horde/imp/index.php:19
  5. Horde_Registry->pushApp() /usr/share/php/Horde/Registry.php:298
  6. Horde_Registry->callAppMethod() /usr/share/php/Horde/Registry.php:1617
  7. Horde_Registry_Application->authenticated()  
/usr/share/php/Horde/Registry.php:1197
  8. IMP_Application->_authenticated()  
/usr/share/php/Horde/Registry/Application.php:108
  9. IMP_Auth::authenticateCallback()  
/usr/share/horde/imp/lib/Application.php:134
10. IMP_Imap->doPostLoginTasks() /usr/share/horde/imp/lib/Auth.php:243
11. IMP_Imap->updateFetchIgnore() /usr/share/horde/imp/lib/Imap.php:344
12. IMP_Mailbox::getSpecialMailboxes() /usr/share/horde/imp/lib/Imap.php:354
13. IMP_Mailbox_SessionCache->getSpecialMailboxes()  
/usr/share/horde/imp/lib/Mailbox.php:1364
14. IMP_Mailbox::getPref()  
/usr/share/horde/imp/lib/Mailbox/SessionCache.php:268
15. Horde_Prefs->getValue() /usr/share/horde/imp/lib/Mailbox.php:235
16. Horde_Prefs->_getScope() /usr/share/php/Horde/Prefs.php:304
17. Horde_Prefs->_loadScope() /usr/share/php/Horde/Prefs.php:397
18. Horde_Prefs_Cache_HordeCache->store() /usr/share/php/Horde/Prefs.php:472
19. serialize() /usr/share/php/Horde/Prefs/Cache/HordeCache.php:62

The error does go away if I delete the impPrefs attribute in the  
existing LDAP-Server.
But this don't seem the best solution looking at the huge number of  
affected LDAP-Users.

I would be happy to here about other strategies :-)

Thanks
Andreas