Hello,
I believe your /customer1 endpoint is working fine, but in the page response, there's some JavaScript that attempts to load /common/NSLogin.jsp?redirect=/console on your server, and it's this URL that is causing the "No such file" errors.
You mention that it works on root, and this would make sense, as then the /common/NSLogin.jsp URL would be proxied through.
So, your solution will require either updating this JavaScript to incorporate the /customer1 into the URL, or else add a location directive to proxy /common/NSLogin.jsp or something to that effect.
I hope that helps a bit.
Best,
Jesse