[horde] Horde *Dav Calendar/Addressbook Error

[Solexiv - Lukas Großberger]

Hi there,

I'm running Horde (5.1.4) on a PLESK (11.5.30) administrated Debian
(6.0.5) root server.
When I visit the URLs that should give clients access to my address book
[1] and calendar [2, 3] in my webbrowser, I get the following error:

A fatal error has occurred
A storage backend for DAV has not been configured.
Details have been logged for the administrator.

But the log file at /var/log/psa-horde/psa-horde.log is empty.

I tried accessing the URLs directly because I wasn't able to integrate
them in Thunderbird.

Does someone have a suggestion on this?
And is this even the right place for this question or did I miss something?

[1] http://DOMAIN.TLD/rpc.php/turba/USERNAME/edas993228rasd832kas34/
[2] http://DOMAIN.TLD/rpc.php/kronolith/USERNAME/a893h4ad22dA.ics
[3] http://DOMAIN.TLD/rpc.php/principals/USERNAME/