The self-service modules are meant to be installed by the AppCenter (and on a dc master or backup). If you didn't install the app, but the packages manual, then that is the reason for the problem. The "permission denied" does not refer to the user, but to the frontend of the software not being allowed to connect to the backend.
Please do the following:
1. install the app (on a dc master or backup):
univention-app install self-service
- you should now have two packages installed:
dpkg -l 'univention-self-service*'
- run the join scripts of both packages (again):
univention-run-join-scripts --run-scripts --force 34univention-self-service.inst
univention-run-join-scripts --run-scripts --force 35univention-self-service-passwordreset-umc.inst
Now it should work. If you still have problems, please check the following log files: