So XMR not starting won’t make any difference to whether you can log in to the web UI or not.
The path it references is inside the container, and should be writeable. It appears something has changed it and made it so it can’t write, so I would down that container and bring it up again to create it fresh, and that should be resolved.
With regard to your other issue, the proxy is saying it can’t connect to the CMS. You need to look at why that might be.
We have a guide that deals with putting the CMS behind a reverse proxy for SSL termination. You might want to check that you’ve done it in that way.