I did that now and calendar and adress book are ok and both on PC and FP.
https://posteo.de/hilfe/kalender-und-adressbuchabgleich-mit-davdroid-android
But I can’t find the tasks except on FP. So where is DAVdroid syncing them to? (on the FP it shows in TASKS the connection from FP tasks to the posteo account, but in posteo there is no possibility for entering tasks).