Mirroring private github repo to gitlab suddenly fails

We mirror a github repo to gitlab, but today the mirror mysteriously stopped working.

All efforts to redo it have failed. When trying to set it up via HTTPS it complains that the remote repo was not found. When trying to set up via SSH it complains that it either cannot add the relevant RSA host key (if host key detection is automatic) or does not have a valid RSA host key (if host keys are input manually).

Was there something that changed recently that could lead to this breakage?

I think we’ve got it fixed. The issue turned out to be that a branch named “user/hotfix” was created on github, mirrored to gitlab, and then deleted on github. Later on, a branch named “user/hotfix/take2” was created and because git has rules that prevent branch names from being subsets of other branch names, this one could not be mirrored to gitlab, with the cryptic error message “Invalid reference name”.

Would be nice if gitlab could provide more explicit error messages. Any idea how to create a ticket for the gitlab folks?