Unable to add Global permissions to groups

Symptoms

When trying to add Global permissions to groups, the following message and screen are displayed on Stash UI:

An error occurred
Something went wrong while trying to serve your request. Try reloading the page.

This message is caused by a reverse proxy set up in front of Stash. Please:

Bypass the proxy
Fix the proxy configuration in compliance with one of the scenarios described on
- Proxying and securing Stash