What version are you running?
3.0.12
What's the URL of the page containing the problem?
https://codereview/r/new/
What steps will reproduce the problem?

Running latest version of GitLab
Go to https://codereview/r/new/
Select a repo
Select a branch
Select an existing commit to create a code review from
Internal Server error caused by trying to access https://gitlab/group/project/commits/hash.diff?private_token=<token>


What is the expected output? What do you see instead?
Review Request should be created with diffs
What operating system are you using? What browser?
Arch, Firefox
Please provide any additional information below.
Recent (11.x I think) changes to GitLab removed access to protected repos in gitlab using personal access tokens, thus meaning Review Board cannot automatically get a diff for a commit automatically.

Issue on GitLab here: https://gitlab.com/gitlab-org/gitlab-ce/issues/56532