I found the issue!
In the Horde docker container the zoneinfo for UTC is wrong.
I have entered the container with univention-app shell -ti horde bash
root@horde-81323206:/etc/horde# cd /usr/share/zoneinfo/
root@horde-81323206:/usr/share/zoneinfo# ls -l UTC
lrwxrwxrwx 1 root root 33 May 29 07:41 UTC -> /usr/share/zoneinfo/Europe/Berlin
UTC zoneinfo is a link to Europe/Berlin. Which does not make sense at all but explains all the symptoms we see.
After replacing the file in the container with the UTC file from the UCS Host the calendar displays events correctly. Also invites are send with correct date and time information.
Univention Staff please have a look:
Horde version 5.2.17-2 using docker container docker.software-univention.de/ucs-appbox-amd64:4.3-0 (0363a55bd2ea)
Is this a general problem of the container or does the install of Horde screw this up?
Edit: added hash of docker image