Normally, the Save folder contains an Autosave folder. If this Autosave folder is deleted (e.g. when deleting old save files), a problem can occur. If Autosave is set to every turn, the problem auto-fixes. If frequency >1, then error messages are created. Also, further errors happen because last-turn files are not made. Log is attached which I THINK shows the problem. Game run on Windows10.
Fixed in git.a5cafea.