If you still use the config from https://selfoss.aditu.de/forum/index.php?id=1400, you have selfoss in the public mode. If you set the password key in the config.ini file to one you got from the /password virtual page, you should be able to enter the password into a login page. You can open it by clicking the key button in the sidebar:

Or you can change to public to 0 and you should be greeted by the login page.