[horde] CalDAV sync OSX: dates disappear, reappear dates by every sync

[Stefan Johänntgen]

I have subscribed to the default calendar in the OSX app "Calendar" via
CalDAV.
When sync appointments disappear, the next sync they appear again, and at
the next sync they are gone, and so on.
In the server log I can not find any error messages. Has anyone had similar
experience?
I use Horde Groupware 5.1.3 and OSX 10.8 and 10.9