My opinion is that this error comes up whenever there's a mismatch of the calendars that are being synced on the N950/N9, and the calendars that are actually available on the CalDAV server at a particular point of time.

Some of the activity in this thread over the last month or so seemed to be related to ongoing issues that were acknowledged by Google.

My own experience with the N950 and N9 is that CalDAV syncing tends to be quite stable for several months at a time, and the oddity of the actual error message is makes this more interesting, but I'm guessing that the internal error handling probably just isn't leading to the correct final error, e.g. calendar X unavailable under account Y.