To any hapless person who comes this way:
I fixed the problem, i.e. I made the problem go away for the moment.
I had deleted a google caldav from my calender list since I was in any case not enabled.
I previously had confusing things happen with multiple caldav's (whatever they are)
I restored the google calendar and left it disabled.
The offending message went away.
Perhaps there were common events on both calendars, but it is not obvious why I cannot delete one without effecting the other.
Nevertheless, the "server responded with '400' .... " message is totally useless to the me, the user, and did not help me resolve the problem.
If there is a conflict, the problem is not solved; I've just returned to the previous ambious state.