OK... I have to have a better read of other comments...
This note regarding '-u hts' directed me in the right direction - I hope...
1) I removed tvheadend completely (including the hts-user and every hts-setting/folder under /root/ and /home/'user'.
2) I installed tvheadend again.
3) Started tvheadend from cli: 'tvheadend -C' -> tvheadend is running as 'user',
lets say user=tom for example
4) open browser and go to '
http://localhost:9981'
Because of option '-C' no login needed.
5) Create a new user e.g. 'root-tvh' and setup password for this user
6) Leave browser and stop the running process 'tvheadend -C' -> focus terminal where you start 'tvheadend -C' and press ctrl+c.
7) Within /home/tom you do have the .hts-folder with the just created credentials/user. As you've started tvheadend as user 'tom' this is where the setup is writen.
Copy it to /home/hts/ with the command: 'sudo cp -r /home/tom/.hts /home/hts'
8) Go to /home/hts and have a look at the folders/files with 'ls -al'.
You will see that the folder .hts is owned by root:root
Change that with 'chown -R hts:users /home/hts/.hts'. Important is the owner 'hts'!
9) Start the service 'sudo systemctl start tvheadend.service'
10) Open your browser and login to tvheadend (see 4)) with the created new user/admin.
11) Deaktivate the (old) default-user, safe and logout.
12) Stop tvheadend.service and start it again.
After this I could login with the new created admin.
