I have found out now, that the server doesn't receive a username at all, whenever I try to sync my calendars. The corresponding field in the server log is empty.

Is it a bug of the CalDAV App on the N9 or a bug of the server?

When I set up the CalDAV-Sync in Thunderbird/Lightning with the same server, it works like a charm.

By the way, the username consists of my mail address. But then, Google calendar doesn't seem to have any problems recognising a mail address as username sent from the n9.

Very strange indeed...