I’ve been able to figure out why the import in Thunderbird failed for the 51 entries:
If the UID of a VEVENT contains “@domainname.tld” or “@machine-name”, the Posteo.de CalDAV server returns HTTP statuscode 404 for the REPORT HTTP method.
I have now removed all occurences of “@followedbyastring” and successfully imported my .ics-file with Thunderbird.
I have not tested the import on my phone using DAVdroid and Calendar Import Export.
Should Posteo.de support occurences of “@followedbyastring” in a UID?
Sorry for the noise, I initially thought this was a DAVdroid problem.