User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.111 Safari/537.36

Steps to reproduce:

I am experiencing the same problem with trying to dismiss reminders with a different service provider.
Calendar events with for a CalDAV synchronised calendar do not be dismissed and continue to pop up.

Thunderbird 78.3.2 (32-bit)
Calendar via: zoho.com
The calendar correctly synchronises events in the calendar.
Workarounds: edit the event to remove the reminder or delete the event
I did not have this problem with Thunderbird v68.x.x and it is not related to a specific CalDAV calendar or account.

Steps to reproduce:
Add the CalDav Calendar: (File>New>Calendar>On the Network>Select CalDAV>Enter username & the location e.g. https://calendar.zoho.com/caldav/<calendar ID>/events >Enter the password)
Add an event (Today) say now + 7 minutes with a reminder for 5 minutes.
When the reminder pops up, try to dismiss (click 'Dismiss' or 'Dismiss All' or 'Snooze for (5min)' or 'Snoose All for (5min)')
The reminder disappears for about 1 second but then pops back up. Similarly if just closing the reminder window, it just pops back up.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1666156
Filed here as it may be more relevant to CalDAV component rather than Alarm.

Actual results:

Calendar events with for a CalDAV synchronised calendar do not be dismissed and continue to pop up.

Expected results:

Event should be cancelled

All these bugs seem to be describing the same problem. Not sure how to mark them as duplicates here so I'm listing them here.

https://bugzilla.mozilla.org/show_bug.cgi?id=1634617
https://bugzilla.mozilla.org/show_bug.cgi?id=1655245
https://bugzilla.mozilla.org/show_bug.cgi?id=1666156
https://bugzilla.mozilla.org/show_bug.cgi?id=1667087
https://bugzilla.mozilla.org/show_bug.cgi?id=1670526
https://bugzilla.mozilla.org/show_bug.cgi?id=1673219