User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

may i did not found the correct component ('calendar'), but as far as my research, calendar is no more an add-on. It is integrated inside thunderbird.
System

• thunderbird 91.1.0
• Calendar synchronized with external cloud service
• to avoid calendar conflicts, this calendar inside thunderbird is write protected
• since a few weeks (may the latest update), the write-protect removes itself after a while. This results in a lot of reminders that can not be confirmed (no option to close them).
• the exactly thing was working since a few years without any problems
• when i enable the write protect (then only reminders appear which i can also confirm/click away), but after a while, write protect is disabled again.

Actual results:

calendar write protect removes itself after a while

Expected results:

calendar write protect should not remove itself after a while

Are reporting that reminders cannot be dismissed?

Sound similar to bug 1729235 but without user interaction (restart Thunderbird, disable/enable calendar).

Yes, reminders can not be dismissed.
And yes, but 1729235 describes exactly the status on my computer.
So these two bugs is the same.
I was searching for similar bugs before creating this one, but didn't found similar ones.