Exchange 2016 calendars broken in Thunderbird 78

Hi

Symptom:
After an update from Thunderbird 68 -> 78.0.1 the calendars stopped working.
They contain no entries anymore.
Un- and Re-subscribing (as CalDAV) doesn't fix the issue.
Re-subscribing as iCal shows entries but add/change/remove actions result in MODIFICATION_FAILED (which is obvious).

Software versions:
Server: Exchange 2016
DavMail: 5.5.1 (was updated from 5.4.0, issue was present there also)
Client: Thunderbird
OS: Mac OS 10.15.6 (replicated issue also on a Windows 10 x64 installation)

The DavMail log shows (TB 78):
...
2020-07-29 17:40:01,506 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:52503
2020-07-29 17:40:01,507 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:52504
2020-07-29 17:40:01,929 INFO [CaldavConnection-52503] davmail.connection - LOGON - 92.75.153.243:52503 tstather@mr.mpg.de
2020-07-29 17:40:02,122 INFO [CaldavConnection-52504] davmail.connection - LOGON - 92.75.153.243:52504 tstather@mr.mpg.de
2020-07-29 17:41:57,672 INFO [CaldavConnection-52504] davmail.connection - DISCONNECT - 92.75.153.243:52504
2020-07-29 17:41:57,674 INFO [CaldavConnection-52503] davmail.connection - DISCONNECT - 92.75.153.243:52503
2020-07-29 18:02:01,758 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:52924
2020-07-29 18:02:01,760 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:52926
2020-07-29 18:02:01,910 INFO [CaldavConnection-52926] davmail.connection - LOGON - 92.75.153.243:52926 tstather@mr.mpg.de
2020-07-29 18:02:02,022 INFO [CaldavConnection-52924] davmail.connection - LOGON - 92.75.153.243:52924 tstather@mr.mpg.de
2020-07-29 18:02:03,840 INFO [CaldavConnection-52924] davmail.connection - LOGON - 92.75.153.243:52924 tstather@mr.mpg.de
2020-07-29 18:02:03,848 INFO [CaldavConnection-52926] davmail.connection - LOGON - 92.75.153.243:52926 tstather@mr.mpg.de
2020-07-29 18:02:09,588 INFO [CaldavConnection-52924] davmail.connection - DISCONNECT - 92.75.153.243:52924
2020-07-29 18:02:09,590 INFO [CaldavConnection-52926] davmail.connection - DISCONNECT - 92.75.153.243:52926
2020-07-29 18:02:22,776 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:52942
2020-07-29 18:02:24,129 INFO [CaldavConnection-52942] davmail.connection - LOGON - 92.75.153.243:52942 tstather@mr.mpg.de
2020-07-29 18:02:24,145 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:52946
2020-07-29 18:02:24,239 INFO [CaldavConnection-52946] davmail.connection - LOGON - 92.75.153.243:52946 tstather@mr.mpg.de
2020-07-29 18:02:34,228 INFO [CaldavConnection-52942] davmail.connection - LOGON - 92.75.153.243:52942 tstather@mr.mpg.de
2020-07-29 18:02:34,238 INFO [CaldavConnection-52946] davmail.connection - LOGON - 92.75.153.243:52946 tstather@mr.mpg.de
2020-07-29 18:02:36,490 INFO [CaldavConnection-52942] davmail.connection - LOGON - 92.75.153.243:52942 tstather@mr.mpg.de
2020-07-29 18:02:36,499 INFO [CaldavConnection-52946] davmail.connection - LOGON - 92.75.153.243:52946 tstather@mr.mpg.de
2020-07-29 18:04:32,590 INFO [CaldavConnection-52946] davmail.connection - DISCONNECT - 92.75.153.243:52946
2020-07-29 18:04:32,592 INFO [CaldavConnection-52942] davmail.connection - DISCONNECT - 92.75.153.243:52942
2020-07-29 18:05:36,701 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:53405
2020-07-29 18:05:36,703 INFO [davmail.caldav.CaldavServer] davmail.connection - CONNECT - 92.75.153.243:53406
2020-07-29 18:05:36,807 INFO [CaldavConnection-53405] davmail.connection - LOGON - 92.75.153.243:53405 tstather@mr.mpg.de
2020-07-29 18:05:36,824 INFO [CaldavConnection-53406] davmail.connection - LOGON - 92.75.153.243:53406 tstather@mr.mpg.de
2020-07-29 18:05:38,643 INFO [CaldavConnection-53405] davmail.connection - LOGON - 92.75.153.243:53405 tstather@mr.mpg.de
2020-07-29 18:05:38,653 INFO [CaldavConnection-53406] davmail.connection - LOGON - 92.75.153.243:53406 tstather@mr.mpg.de
2020-07-29 18:07:33,882 INFO [CaldavConnection-53405] davmail.connection - DISCONNECT - 92.75.153.243:53405
2020-07-29 18:07:33,884 INFO [CaldavConnection-53406] davmail.connection - DISCONNECT - 92.75.153.243:53406
...

A downgrade to TB68 made the calendars work again, something must have changed in TB78

Best,
Thomas