Thunderbird-Hilfeforum

I'm using an old, no longer supported add-on called Reminderfox that saves its contents in ics format. I used to be able to import this to Google calendar and Thunderbird but for some reason they and also Microsoft Outlook won't now import the ics file. They either give an error as in Thunderbird's case (error, no items to import) or just remain greyed out so I can't do anything else as in Google and Microsoft's case. I have submitted the ics file to an ics verifier online and it says there are no errors. I have also looked at the file in notepad and can't see any obvious errors, although I'm not fully conversant with the ics format. Can anyone suggest what the problem might be or anything else I can try please?