Hurray! I eventually found my contacts. Not very intuitively I had to open “Contacts”, select “Settings” and “Filter” to get to a screen where the contact providers (SIM, Google, DAVdroid, etc.) can be enabled:
Do you think that it is possible to enabled DAVdroid contacts when adding a DAVdroid account? That would be much easier for newbies like me.
Thanks and sorry for bothering you.