500 error for a specific commit

We use GitLab in https://gitlab.com/.
When I checked an old commit of a project from ‘Commits’, the page returned 500 error.
This commit used to be visible but today I can’t see it at all, and as I checked other commits around it, I couldn’t find any commit which returns 500 error.
I don’t know why only this commit can not be opened.
Other features, such as pulling and pushing work by the way.
This is not a public project but I wrote down the URL for staff.