New merge request for a fork fails with 503 error

Trying to open a MR for a small code change via the "Create merge request" button:

https://gitlab.com/PhilLab/gitlab-ce/merge_requests/new?merge_request%5Bsource_branch%5D=patch-31&merge_request%5Bsource_project_id%5D=9218143&merge_request%5Btarget_branch%5D=master&merge_request%5Btarget_project_id%5D=13083

yields a "503 - Whoops, GitLab is currently unavailable". Tried it multiple times