Apparently the problem was that somehow the git tree for 3 of my 4 repositories was... incomplete? If I removed all remotes but my bup repository the message disappeared but `git annex sync` still failed with a "fatal: refusing to merge unrelated histories", which should have been fixed in April. I decided using `git fetch --all` for all repositories, manually merged with the `--allow-unrelated-histories` option, uninit'ed all the repositories and rebuilt them from scratch. The "fatal: refusing to merge unrelated histories" appeared again since I have several disconnected repositories with more or less the same set of files but I knew how to resolve that.