have you restarted Thunderbird since it was updated. If not do so.

I've closed Thunderbird and reopened it but that doesn't help. Neither does rebooting my computer.

in Troubleshooting information on the help menu.

Look at the Version in application basics. Note it. Scroll down to the extensions heading and locate lightning note the version.

Are they the same? What are they?

Version in application basics is 68.3.1 Version in extensions heading is 68.3.1.1 Now what?

Is this a local or network calendar, and if it's the latter, who is the provider, and which add-on and version are installed to sync the calendar?

Just me and my calendar. No network.

Hi, The symptoms you describe sound like it's the same problem as under discussion here. We think the problem seems to occur when loading the add-ons after a restart. See in particular the most recent posts on page 2 regarding use of TB "Safe Mode" (with add-ons disabled). This problem is also the subject of a bug filed here. If your problem turns out to be the same, please add any info you can to the bug discussion.

Still no luck. I've restarted in safe mode. Downloaded new version of lightning (64.4.1.1). It works once then not again after I close TB and then reopen it. I'm a luddite when it comes to most things involving computers . So simple instructions would be best, assuming there is an answer.

Hi Lbarton What you report seems to confirm that you’re experiencing the same problem as me and some others. There is no fix that I know of, but there’s a sort of workaround. You will have seen in the other thread Paulo’s suggestion: When, having launched TB, you find that the calendar is not working, do the following: 1. Go to the Help menu > Restart with Add-ons Disabled... 2. Continue with Safe Mode... (don’t tick the boxes in the dialogue) TB will restart with all the add-ons disabled. All you have to do then is 3. Exit TB 4. Start TB normally

The calendar should work until you exit TB. When you next launch TB you’ll have to go through it all again.

I know this sounds weird but it seems that going through safe mode cleans up something that then goes wrong each time we exit TB. This is why we keep having to go through safe mode before we can use the calendar again.

Personally I am limiting the problem by never logging out from Windows, I just use hibernation or sleep, that way I don’t have to exit TB so often.

Tell us if this workaround allows you to use the calendar.

Hi John Thompson, Oh sigh! but thanks for the "solution." Just another little challenge to deal with. Might be easier to go back to paper calendars!

I found where the problem was coming from, you likely just need to change a setting. https://support.mozilla.org/en-US/questions/1273975