Git repository not accessible due to "Unable to complete your request, The following exception occurred: null"

When you try to open any asset in a given repository, this ends with

"Unable to complete your request, The following exception occurred: null"

It makes the whole repository inaccessible, and the only current remedy we have is this:

$ git clone system.git/
$ cd system
$ git branch -a // find out the user's branch
$ git push origin --delete <USER_BRANCH>

This error only happens for a specific user - if you log into different user, you can access the same resources without any errors.

Unfortunately, we do not have a consistent way of reproducing.
Let us know what could help in investigation.