I guess it was related. It works again with 188.8.131.52.
More details on my setup with Davdriod V1.0.9:
I use Baikal 0.2.7
Android 6.0.1 on a Samsung SM-G935F (S7)
Downloading calendar data to the phone works, but upload would fail with the HTTP Response from above:<s:exception>Sabre\DAV\Exception\UnsupportedMediaType</s:exception>[LF] <s:message>This resource only supports valid iCalendar 2.0 data. Parse error: Invalid VObject, line 1 did not follow the icalendar/vcard format</s:message>[LF]
Let me know if you need more details.