Server Error 500 on opening MR

Summary

I keep getting a 500 server error on gitlab.com when opening a MR against the master branch gitlab-org/gitlab for the branch behrmann/gitlab:fixsourcepages.

Steps to reproduce

1. Open the the PR either by clicking the "New merge request" button at the GitLab repo or via the link or via the link after pushing the branch via SSH ("To create a merge request for fixsourcepages, visit:")
2. When creating the MR from the "New merge request" button choose behrmann/gitlab fixsourcepages as source branch and click "Compare branches and continue"
3. Choose either no templare or the documentation template, fill it out and click "Create merge request"
4. Wait.
5. Get a 500 and the dreaded "Whoops, something went wrong on our end." page

I have reproduced this both in Firefox and Chromium.

Example Project

This problem occurs on gitlab.com, with the gitlab-org/gitlab repository as target and behrmann/gitlab:fixsourcepages as source.

What is the current bug behavior?

Upon clicking the button to open the MR, wait for a long while and finally get a 500.

What is the expected correct behavior?

Opening the MR.

Relevant logs and/or screenshots

Just the generic "Whoops, something went wrong on our end." page.