I have the same problem with smartphone Leagoo Elite 5. It’s work with caldav sync adapter but not with davdroid.
Handle VCard/3 and iCalendar encodings
VCard/4 always requires UTF-8, but VCard/3 and iCalendar resources may use other charsets/encodings than UTF-8.
So, the Content-Type headers should be parsed for charset information.
When sending, DAVdroid always uses UTF-8.