Horde Error retrieving free/busy information

[JIKOmetrix]

Hi,

I'm trying to use Horde as my web mail and calendar. When I add an attendee to an event I get this error.

Notice: Only variable references should be returned by reference in /usr/local/cpanel/base/horde/kronolith/lib/Storage.php on line 55

Message Error retrieving free/busy information for techone@buddemarketing.com: Not found

I have added the follow free/busy URL to the address book of the attendee I want to add:
https://domain.tld:2096/horde/kronol...ser@domain.tld

I still get the error. every time I open the attendee window to add someone I get the error again.

Anyone able to get this working? Have any hints?

Mike