Problem solved. It was the OpenTasks app.
SOLUTION: In the permissions settings of the OpenTasks app the option “Contacts” (in german “Kontakte”) has to be turned on.
Thanks for your help.
I am using DAVdroid fo sync my Android contacts against a horde groupware server.
After starting the DAVdroid app on the smartphone I see 2 addressbooks with the same name. But I have only 1 addressbook on this account on this server …
Upgrading to Horde Framework 5.2.14 solved the problem.
Thanks for the tip.