This is a known issue and fortunately, there is a workaround to get things back to normal. Proceed as follows:

1. Quit Thunderbird
2. Make a copy of the local.sqlite file and place it on the desktop or some other convenient location.
3. Rename the original file (the one inside the calendar-data folder in your Tbird profile folder) to local.sqlite.bak
4. Install the SQLite Manager extension for Chrome or the add-on for Firefox available here.
5. Click the icon of the extension or add-on to launch it. It should open in a new tab.
6. Drag the copy of the local.sqlite file made in step 2 above and drop it into the SQLite Manager tab in your browser, or use the File menu at the bottom of the tab to open it.
7. Clear the yellow field where the cursor is then paste the following query inside it:

SELECT * from cal_events WHERE id IS NULL LIMIT 100

Press Enter to execute that query. The output should return rows of your calendar events which are not being displayed in Thunderbird. If that is the case, run the following query in the same way:

DELETE FROM cal_events WHERE id IS NULL

Save the file using the Save menu at the bottom of the SQLite Manager tab. The saved file should download to your Downloads folder. Locate it and place it in the calendar-data folder inside your Tbird profile folder then launch Thunderbird. If all goes well, your calendar events should be visible in Thunderbird once again.

Thank You Stans!

Thanks but I do not have a calendar-data folder on my computer. I found one with the local.sqlite file but they are dated 30/09/2015 on a backup hard disc which is connected.

Thanks but I do not have a calendar-data folder on my computer. I found one with the local.sqlite file but they are dated 30/09/2015 on a backup hard disc which is connected.

Are you looking inside your Thunderbird profile folder? Profiles - Where Thunderbird stores your messages and other user data

Thanks for that, I found my profile folder and followed the instructions with a good deal of intrepidation and skepticism but in the end is worked and I have my events back. Thank you again

I... followed the instructions with a good deal of intrepidation

That's why I included step #2 and #3, so that the original file stays untouched while you work with a copy. It's always a good thing to have a backup of data you treasure and I hope you do regularly backup your data. Not even the industry bigshots like Microsoft can guarantee that a bug in their product releases (e.g Windows 10) will never result in loss of data, so now that you know where Thunderbird stores your data, there's absolutely no reason for not having that data backed up somewhere. Case in point https://support.mozilla.org/questions/1353242 and many others like that. Data loss is a painful and expensive experience. Those that have gone through it do not need to be told twice about taking measures to avoid it.

Went through all that, still no calendar items found. It looks like everything got deleted from the V22 file yesterday, and the V20 on my back-up is over a year out of date. Repeated with all available local.sqlite files - still zero calendar items found. Back to paper and pen diary, then.

That seems to have worked for me. It seems like magic, so clearly this is a sufficiently advanced technology.

I'd be interested to know what was happening, though. Where does the "id is NULL" entry (there was just one) come from? Is it seen as "end of file" by the calendar?

Really scary suddenly not knowing what I've said I'll be doing for the rest of the week. Yes I'd backed everything up but I realised I had no idea how to actually read the calendar data. In future I'll export it before updating Thunderbird.

This past two days I found my Tbird had updated to ver 91. It killed some of my imported calendars that are very important for my biz use.

I followed the steps above from Stans, but am not getting any error like on the screenshot when I ran it thru the SQLite Manager as advised. File size remained same and it appears I may have a different issue and nothing was changed.

I'm on Win 10 20H2 OS Build 19042.1237 on Dell XPS 8920 i7 CPU with 16 GB RAM.

This is a Tbird that has been updated for many years and could probably be in serious need of a new fresh start but hate to lose any of the decade plus of info it holds both biz and personal.

Would unsubscribing the calendars and resubscribing be possible answer? Or will I lose past history results?

Any help or ideas would be greatly appreciated. Thanks in advance! Kelly

OK... I see there is update to Provider for Google Calendar and that seemed to restore missing imported calendars... Whew... took 3 hrs plus to think to check this. Hope it helps other out.

Think I'm good to go now. Kelly

First, apologies to sfhowes: the date 10/8/21 in the original question is, to me, 10th August 2021 so I assumed it referred to an upgrade in August, not this week's upgrade.

Secondly, the 'chosen solution' is way beyond my computer abilities. I have no idea of what a local.squlite file is nor where I might find it. The recommended solution is probably perfectly simple to a computer technician but to someone who is simply a Thunderbird user, it is daunting.

Is there any possibility that there will be an upgrade to the upgrade to sort this? If not, I'm going to buy a paper diary and give up using the Thunderbird calendar/events/tasks options.

The script above did not work for me, but reinstalling 'Provider for Google Calendar' solved the problem instantly. Reinstalling 'Lightening Calendar Tabs' restored the tabs that had been lost, which are now in glorious technicolour.