Mirroring to remote repository fails after a force-push

I attempted to use the mirroring setting at the "ocserv/ocserv" repository in gitlab.com. Everything went well, up until a feature branch on which I was often rewritting history and force pushing. The mirroring started outputing errors such as:

The remote repository failed to update 4 minutes ago. Last successful update about 2 hours ago.
The following branches have diverged from their local counterparts: travis

I then attempted to remove the problematic branch from the remote repository hoping the issue will be solved. However then the error message become larger and it is now:

The remote repository failed to update 4 minutes ago. Last successful update about 2 hours ago.
The following branches have diverged from their local counterparts: travis

error: unable to delete 'travis': remote ref does not exist
error: failed to push some refs to ...