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)Favorite marks were gone and did not reappear. (Favorite list was empty) Linked contacts lost several fields of information (e.g. phone number or email address) in the posteo web interface, e.g. for 2 contacts only the name of the contact remains available in the posteo web interface. I could not find some contacts at all in the posteo web interface during the sync procedure. The Android contact app is now the only place to show all contact information. In edit mode each of the problematic contacts (see point 2 above) is shown as linked to several other contacts with the same name from the same posteo default addressbook. Also one is just linked to a WhatsApp entry.
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)