I have the same issue as petzi43. Working with openproject in 6.1.5 in a VM (google).
Self-registration is set as "disabled," and I cannot change it. I've tried restarting everything and logging in and out.
To doublecheck that it wasn't just a interface error, I would change it to one of the other options, and send myself an invite. Without fail, when clicking the invite link in the email, all options result in openproject throwing the error: "User registration is disabled on this system. Please ask an administrator to create an account for you."