Git error during push: `cannot lock ref `

During a push to gitlab.com via HTTP I got the following error:

Counting objects: 6, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 744 bytes | 0 bytes/s, done.
Total 6 (delta 5), reused 0 (delta 0)

remote: error: cannot lock ref 'refs/heads/api-cleanup-entities': Unable to create '/var/opt/gitlab/git-data/repositories/gitlab-org/gitlab-ce.git/refs/heads/api-cleanup-entities.lock': File exists.
remote: 
remote: If no other git process is currently running, this probably means a
remote: git process crashed in this repository earlier. Make sure no other git
remote: process is running and remove the file manually to continue.
To https://gitlab.com/gitlab-org/gitlab-ce.git
 ! [remote rejected] api-cleanup-entities -> api-cleanup-entities (failed to update ref)
error: failed to push some refs to 'https://gitlab.com/gitlab-org/gitlab-ce.git'

@pcarranza @jnijhof Maybe the operations team can look into that.