User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:90.0) Gecko/20100101 Firefox/90.0

Steps to reproduce:

1. I set up my work e-mail, which uses gmail as its provider. I synchronized calendars, reminders, etc.
2. When everything was synchronized, it loaded my mail, and presented me with a bunch of reminders (most old reminders)
3. Clicked on "Dismiss all"

Actual results:

It seems that thunderbird is failing to write to the calendar (which may not be a bug in itself), but I can't get rid of the popup. Whether I close it or try to dismiss things, the popup will close, but immediately show again with annoying sounds. No clue how to fix.

Expected results:

The popup should not keep showing up. Thunderbird should point me to some resolution after it is not able to write to the calendar.

Are you using the google calendar through CalDAV or the add-on?
Anyway, reminders that don't want to dismiss is not a new issue. There are many bug reports, but I think mostly it's not well understood why it happens.

I do believe, the fix for this has been uplifted for the next next point release of 91.

Martin, did you really experience this issue with Thunderbird 91.0? Are you using the Google Calendar through CalDAV or the add-on?