From the same server I can successfully sync cardDAVs contacts, and IMAPs-mail, so I've ruled out network issues.
Furthermore, that same installation also syncs many other users (non-i) -Devices Contacts, Calenders, and Mails, so I've ruled out SOGOs CalDAV implementation.
- Rebooting normally
- Signing out of iCloud, then rebooting
- Factory-reset, then configuring the CalDAV first, then the rest.
- Opened the non-SSL Port (shudder) to the outside world for testing.
- Creating the Calender "twice", Once as Calender, and once as Contactlist
None of those worked, and the last one was impossible through the settings dialog
I'm not quite sure, how to proceed. In the meantime, I bought some cheap androids, so that my users at least have their calendar on the road with them.