We mirror various Github repositories on our Gitlab server to ensure their availability. However, last Friday mirroring suddenly stopped working. I tried clicking the update icon for a manual update, but the error persisted, even when I deleted and re-created the mirror operation (under Settings → Repository → Mirroring Repositories).
This is the error message:

I can delete and re-create the mirrored repository, and the initial data transfer from the remote repository works, but eventually the error re-appears.