That sounds logical, but I'll repeat what I THINK I said initially: If, after I see the error message (which I can't find at the moment), I open the \rainlendar folder, Default.ics is not present, though Default.ics-bak (which ordinarily is not present) is. Doesn't that mean, NOT that another program has locked Default.ics, but that there's some failure -- I suppose a random failure -- within Rainlendar? BTW, this problem began when I was using v.2.10. This last time, it happened with 2.11. FWIW, I still really like Rainlendar.

Well, what actually fails is the rename operation when the original file is replaced with the new one. At that point the old file should have been already deleted. There are no errors related to the deletion so I have no idea what could cause the rename to fail if the target has been successfully deleted. Unfortunately there is very little I can do about this. I would have to somehow be able to reproduce the error here but that is quite difficult if it happens only randomly in your system and has never so far happened in mine.