Im facing a problem with Thunderbird where some calendar events are being deleted spontaneously. I observed that, right before it gets deleted, i see a flashing reminder window and the red bell over the event. I found a thread from May 2021 called "Lightning calender reminders problem" where the contributor Toad-Hall asks for additional information to nail down this bug (1683321). I have a specific scenario here where i can reproduce the problem very consistently: Thunderbird 91.6.0 Portable Windows 10 Pro CalDAV calendar accessing an Office 365 calendar via DavMail Gateway and EWS protocol. 1) Start Thunderbird 2) Open Calendar and observe that one specific event is present (ok). 3) Right click on calendar and execute "Synchronize Calendars" 4) Right after the execution of this command, i see a flashing reminder window and the red bell over the event, and then suddently the event gets deleted. 5) Close and start Thunderbird again. The event is back on the calendar (until the next sync) During all this process, i checked the 365 calendar on the web (browser) and the event is always there (status = confirmed). What kind of information (logs) could i provide for you to investigate this problem?