@Billycee Based on the error description, it does seems like the response came from the web application not from the VPN server itself. So, when you access the base URL directly from the office network, do you see the expected webpage?   Do you use just the hostname as the base URL (http://xyz.com) or appending any directives after the hostname (http://xyz.com/test...) ... View more

Win2016 server should be supported, do you see any error like "Internal state error occurred" after quite sometime, lets say about 15 to 20 seconds later?   please take a tcpdump and see if we are receiving reply from the win server. ... View more