Gitlab Mirror fail to create a branch <branch name> invalid reference name <commit sha>

Question

We have GitHub repositories mirrored to Gitlab. At some point we experienced this error on pull mirroring:

Error: Failed to create branch 'foo/bar/feature-xy': invalid reference name '<commit_sha>'.

Answer 1

The error happened because the Gitlab repository already has a branch called foo/bar, so now Gitlab doesn't allow any new additions that start with foo/bar/*.

In our case, we deleted the branch from Gitlab since, in GitHub, the same branch was already gone, and for some reason, Gitlab doesn't sync.