Since I forgot my password, I removed the “user” and “password” elements from the config.xml file.
The GUI then warns me, that there is no password set, which is to be expected.
But if I enter a new user/password combination in the GUI and save it, the config.xml file isn’t updated, although I don’t get any error messages. I still get the warning and don’t need a password to access the GUI.
I tried to restart Syncthing in the GUI and directly using systemctl.
It works fine now. No idea what went wrong. I replaced the GUI part of the config.xml with one from another system. After that I could reconfigure the GUI settings in the GUI itself. Maybe a typo. The syncthing user had/has access to the file.
Another idea is, that I replaced the SSL certificate with my own. But the link target was empty and Syncthing wanted to recreate it’s own certs, but couldn’t because of the links…