A few users noticed trouble deleting entries, particularly appointments, since yesterday. We tracked it down to Google's new library and found a spot where, if their HTTP server issues a redirect, some necessary information is lost when it retries, causing the operation to fail. It is pretty rare, as it only affects deletion and only if there is also a redirect.
We made a fix and built the Google libraries ourselves, which we deployed a little bit ago. So far we have not seen any similar messages in the logs and we are no longer able to reproduce the problem.