Just wanted to share a silly mistake that I made when testing NAV 2016 web client (could have been any version).
When trying to access the webclient I got the above error (<sessionState mode=”InProc”/>).
I forgot to add the port number in the url. The webclient runs on port 8080 (default), but I didn’t specify this when typing in the url. I was trying to access https://nav2016.yourdomain.com/INSTANCE_NAME/WebClient/ instead of https://nav2016.yourdomain.com:8080/INSTANCE_NAME/WebClient/.
The reason this has given me such a headache is that the url https://nav2016.yourdomain.com/INSTANCE_NAME/WebClient/ does exist, but the web client is configured for port 8080 which is why I am getting the above error.
I hope this will help someone else save some time 🙂