Our authentication and authorization is in handled with a Shibboleth IDP and after updating to 15.8.0 on our test instance after login we are redirected to gitlab.hostname/api/v1/instance which results in a 404. If I delete the “api/v1/instance” from the URL then the home page loads as expected. I’ve looked through the configuration and can’t see where this “api/v1/instance” is being specified.
Turns out the problem was with my browser, wiping cookies and stored data allowed me to login again. I wasn’t able to figure out what exactly was the problem as clearing cookies for GitLab and the IDP didn’t help but starting with a fresh browser profile did.
Do you still see the issue in a private browsing session?