Reverse Proxy Error no user/password was provided for basic authentication

I've got Nginx set up on a RPi as a reverse proxy using SSL between the remote user and the Nginx instance. All seems to work well for two services mounted on the RPi (Shellinabox and RPi Monitor). However I can't get Nginx to work with a Couch Potato instance that is held on another server on the same home network.

When on the home network, I can access Couch Potato from any device on the network without authentication, but when trying to access it externally through the Nginx reverse proxy, the Nginx error log shows:

2015/02/05 10:43:46 [error] 30557#0: *1 no user/password was provided for basic authentication, client: XXX.XXX.XXX.XXX, server: , request: "GET /couchpotato/ HTTP/1.1", host: "XXX.XXX.XXX.XXX"


and the Nginx generated access log for Couchpotato shows:

XXX.XXX.XXX.XXX - - [05/Feb/2015:10:43:46 +0000] "GET /couchpotato/ HTTP/1.1" 401 590 "-" "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)"


The Nginx configuration file in sites-available is here: http://paste.debian.net/144279/

Having read many of the other questions about this I'm guessing that Nginx is trying to pass it's own authentication to couchpotato when I think I don't want anything passed, but I don't know.

Grateful for any help