You have been right. Posteo verified that the error is caused by the broken event. After I have removed the event synchronization works now.
Thanks for your support!
I’m not if this is the best place to post, but https://davdroid.bitfire.at/configuration/ has a link to the forums, so here goes…
DAVDroid also works with the Xandikos CalDAV/CardDAV server (https://www.xandikos.org/). Xandikos supports discovery, so set up is straightforward; specifying a base URL, username and password is enough.
An example howto of migrating from Google contacts to Xandikos, DAVDroid (and vdirsyncer/khal on desktop) can be found at https://www.chiark.greenend.org.uk/~cjwatson/blog/new-address-book.html
Thank you very much for telling us, it’s definately helpful to post it here, because lots of DAVdroid users read these forums! Do you maybe have an instance running where you could make us a test account so that we can test it on ourselves?
There is a test server that I’m happy to give you access to, but since Xandikos is also a generic WebDAV server I’m a little hesitant to post credentials on a public forum.
Is there an email address I send credentials for a test server to?