Hi,
sorry for the trouble here. I was able to reproduce the problem and found a possible error message which could help. During my research on the issue tracker I have found an issue already being debugged and added my findings there: https://gitlab.com/gitlab-org/gitlab/-/issues/242061#note_404692820
It seems that Gitaly handling the Git requests is not passing or using the oauth token properly, but this is just a blind guess of mine. For future analysis and updates, please subscribe to the issue notifications at the bottom right.
Cheers,
Michael
Edit: My assumption was wrong, this turned out to be a configuration problem. The real problem is a security sanitizer fix which was too aggressive.