Repository Cleanup 500


I just worked on purging big files from our repository history. I followed: Reduce repository size | GitLab and all steps, except last one in UI worked well.
When I try to do Repository Cleanup, page loads for a moment (I believe upload of our 300kB commit-map) and than it simply throws me 500.

Tried multiple times, but result is same. Is there anyway to debug it to reduce size of repo?