Reverse Proxy rule that fails with "Proxy Error" The proxy server received an invalid response from an upstream server.

We have an reverse proxy rule that fails but it works in TMG with no issues.  It communicates on TLS 1.0 internally (going thru TMG) and T>S 1.2 externally (going thru Sophos) which I do not think to be the issue but we will receive a 502 proxy error when attempting to access it externally.

There is no Firewall profile set on the reverse proxy rule.  The log contains the following:

2017:01:17-06:39:09 bdolxputm-1 reverseproxy: [Tue Jan 17 06:39:09.266715 2017] [proxy:error] [pid 9046:tid 4130392944] [client XXX.XXX.XXX.XXX:61994] AH00898: Error reading from remote server returned by /notifier.svc
2017:01:17-06:39:09 bdolxputm-1 reverseproxy: id="0299" srcip="XXX.XXX.XXX.XXX" localip="XXX.XXX.XXX.XXX" size="403" user="-" host="XXX.XXX.XXX.XXX" method="GET" statuscode="502" reason="-" extra="-" exceptions="-" time="106081" url="/notifier.svc" server="REF_RevFroDevNotifie" referer="-" cookie="_ga=GA1.2.1864138687.1483970959" set-cookie="-"

I have tried many different settings but do not appear to be able to this particular one to work.

This application resides on the users workstation which will notify the user different information.  i.e. reboot needed, time entry notification, updates needed etc.