Are you importing the .ics file to a local calendar or to a network calendar (e.g. CalDav)?

I found a workaround but it's not very good. Create and .ics file, import it into a Google calendar, export into an .ics file and import the .ics file into Thunderbird. The problem is that the import will fail if you try importing into an existing calendar. It will work only if you import it into a new one.

Tom, to answer your question: the problem is with a local calendar. It happens even when I export and try importing to the same calendar. Has nothing to do with networks.

Thank you for the advice. I have not used Google calanders but might try your work-around. In the past I had no problem in updating Thunderbird calanders on several different systems by just exporting and importing. If this is no longer possible, it seems to be a serious flaw, making Thundirbird no longer a useful tool. Will Thundirbird be updated to correct this fault? It would also be helpfull if Thunderbird updated, as it used to, without having to re-install.

I wish that someone at Mozilla paid attention and corrected this problem. Hopefully the next update.