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 use oxygen Android and sync with DAVx⁵ some address books with my nextcloud.
If I want to save an address I can’t decide the address book.
Which contact app is able to this?
Could be possible that the address book account has never been created yet, so there’s nothing to select.
Also, can you please provide the DAVx⁵ debug info?
there are various chinese manufacturers that seem to patch out the possibility of choosing a specific address book for saving the contacts. You can always use different contacts apps. For example Google contacts or Simple contacts.
Does that help?