I am seeing this now with a CalDav calendar and Thunderbird 78.3.1
Relevant part from error console
```
Lightning: CalDAV: Unexpected status modifying item to RH: 403
BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Europe/Warsaw
Happy to provide further output / test things if it would be useful!
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https://bugs.launchpad.net/bugs/1846423
Title:
Lightning: Cannot dismiss reminders. Got a VALUE parameter with an
