I see a similar issue in Backups n8n Workflows, that wasn’t solved actually. It seems there is some issue with the self-hosted instance accessing itself.
Setting http://localhost:5678/api/v1 as baseUrl has the same problem (Authorization failed - please check your credentials), so I am not sure it is caused by proxy. I assume it is able to reach the instance, since the error is different if it is pointing to a non-existent/other url
Playground works fine for me with the correct key, doesn’t work with any other