[imp] Preferences not saved

[whisky]

Dear list members,

I've already searched the archives but I couldn't find a solution to the
following problem.
When I try to either modify the default identity or to add a new one simply
nothing happens.
I am using the latest versions of horde and imp. Preference System is set to
"SQL Database" with "Horde defaults".
Imp is used for authentication via POP3. The system runs on a Debian box
with MySQL 4.1.11, PHP 5.0.4 Confixx installed (not happy with Confixx, but
that's the way it is...) In the "Database" menu point I've accordingly
selected MySQL improved (4.1+).

When I hit the Save-button within the "Options"-Menu, mysql.log reads:

----------------------------------------------------------------------------
----
892938 Connect     horde at localhost on horde
892938 Init DB     horde
892938 Query       SELECT datatree_id, datatree_parents FROM horde_datatree
WHERE datatree_name = 'horde' AND group_uid = 'horde.perms' ORDER BY
datatree_id
892938 Init DB     horde
892938 Query       SELECT datatree_id, datatree_parents FROM horde_datatree
WHERE datatree_name = 'horde' AND group_uid = 'horde.perms' ORDER BY
datatree_id
892938 Init DB     horde
892938 Query       SELECT datatree_id, datatree_parents FROM horde_datatree
WHERE datatree_name = 'imp' AND group_uid = 'horde.perms' ORDER BY
datatree_id
892938 Init DB     horde
892938 Query       SELECT datatree_id, datatree_parents FROM horde_datatree
WHERE datatree_name = 'imp' AND group_uid = 'horde.perms' ORDER BY
datatree_id
892938 Quit       
892939 Connect     horde at localhost on horde
892939 Quit       
----------------------------------------------------------------------------
----

The very first time I tried to add a new identity, mysql.log showed me this:
----------------------------------------------------------------------------
----
050705 11:14:57  892841 Quit       
                 892840 Quit       
                 892842 Connect     horde at localhost on horde
                 892842 Quit       
                 892843 Connect     horde at localhost on horde
                 892843 Quit       
----------------------------------------------------------------------------
----

As you all can see there's not even an INSERT statement in any of the log
excerpts. 

Does anybody have an idea why that is? Any ideas are welcome.

Thanks in advance,

Whisky