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!
After the last reboot I noticed all my posteo contacs were gone on my phone.
They started reappearing after a while (most likely with the server sync). But looking closer I noticed several effects: (I doubt the reboot is the root case for all issues)
To me it looks like I need a Davdroid Workaround app for Android 7 (the existing workaround app is not shown in the Google Play store) and linked contacts need a bugfix to sync correctly (at least to posteo).
I know this is not a deterministic way to reproduce the issue. But I do not want to try out any more and loose more contact information (especially since the server backup does not seem to be safe). But the issue looks serious enough to me to post it anyway.
Thanks for your help
Version: 1.6-gplay (149) Mon Jun 05 15:01:27 GMT+02:00 2017
Installed from: com.android.vending
Android version: 7.0 (NPN25.137-33)
Device: Motorola Moto G (5) Plus (potter_n)