Cannot dismiss certain reminders in lightning/thunderbird

Found a workaround. Turns out if I uncheck the "Show missed reminders" under Preferences>Calendar>Reminders, I don't haver this issue. I don't need to be seeing missed reminders so problem is (essentially) solved.

The case of the Exchange (EWS) calendar

One issue like this has been running for years in Mozilla support site.

I have been suffering the same problem in my Thunderbird Calendar with the corporate Exchange calendar. In my case, I had removed the recurring event from the calendar but it still was present in my Thunderbird view. I could not dismiss the reminders for the last four occurrences and I could not delete the event either in Thunderbird calendar. In the Exchange calendar (web interface) the event was not present any more.

What seems to have worked out was fiddling with the Thunderbird's calendar settings for this exchange calendar:

• right click on the calendar in the calendar list
• choose "Exchange (EWS) properties"
• In the dialog go to "Folder properties" tab
• Around the middle there is a list of users that, in my case, contains "Default" and "Anonymous" and switched between both a couple of times, watching the permissions change
• Go to "Offline Caching" tab
• click on "Clear offline cached data"
• Additionally I went to "Automatic Processing" and toggled one of the checkboxes twice (effectively leaving it in it original state)
• click "Save and Reset" button
• the problem was solved for me.

The following describes how to still turn off missed reminders, after it disappeared from the options-menue:

https://dwaves.de/2020/04/15/thunderbird-calender-reminders-keep-popping-up-do-not-show-reminder-of-passed-old-missed-events/

tested with: thunderbird 68.6.0 (64-bit) on GNU Linux Debian 10

if one uses the build in calendar of thunderbird (it was called sunbird, was replaced by lightning)

go into advanced config editor: (about:config)

type: calendar.alarms.showmissed

and double-click on the entry

to change it to false

https://github.com/Ericsson/exchangecalendar/issues/546

then simply close thunderbird and restart it