exactly how is this calendar in Thunderbird? Network or local, ics file etc (what provider)

My guess is the status is changing because the source is read only. So first we need to establish where the source is and what kind of storage and protocols are being used.

The Thunderbird calendar is an ics file on the ssd hard drive in a Windows 10 computer. Hasn't changed since I got the computer a couple of years ago. The problem just started recently with the first reminder notification from a monthly task. So it probably started with the latest update.

Thunderbird's calendar is not an ICS file. It has been an SQLite file since the calendar was integrated and the lightning calendar was replaced. (I don't really use the calendar)

So lets go back and try this again. Go to the calendar tab and right click the calendar and select properties. Where does it tell you the calendar is located? Or does it skip straight to the associated email address.(In which case it is a local SQLite database.)

All my 3 calendar files are ics files. This is what the property dialogue box shows as the location of the calendar that has the problem : file:///D:/My%20Documents/Personal/Home.ics The other two calendars don't cause trouble possibly because they have no recurring tasks. I've been using Thunderbird about 4 years now and the calendar has always been integrated and I'm on version 91.2.0 which I suspect might be the problem as it just upgraded recently. Also a correction on what I said earlier the calendar file is on my regular hard drive not my ssd although I doubt that would matter. I suspect it is an ics file because I think I probably imported it from my windows live calendar when I got Thunderbird.