User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

I was recently updated to 91.0.1 and lost all my calendars. I reconfigured all my calendars without using any addons. I have one calendar that is an outlook.office365 account. My others are gmail calendars. The gmail calendars work fine. When I try to add an entry on the outlook.office365 calendar I get a black and white triangle next to the calendar name with the message "The calendar xxx is momentarily not available". I also get a pop up suggesting that I can set the calendar to read only to avoid this in the future if it is not required.

Actual results:

I checked the settings for the calendar and found that it is already checked as "read only" although it was not set as "read only" before attempting to add the calendar event. If I go into Outlook and create an event it works fine and the event shows up in Thunderbird after a resync. I have confirmed this happens on three different computers in the same manner.

Expected results:

I should be able to add and/or edit calendar events directly within Thunderbird calendar and the calendar should not be set to "read only".

Error message that displays when you try to add a calendar event

Office365 does not support CalDAV, or any form of writable calendars (through standard means), AFAIK.
I suppose that calendar is an .ics calendar in read-only format that you subscribed to?
There's not much we can do about that. On creation we even now try to automatically set to read-only if we can detect that.

I don't think there's anything to do here.