After a bit of research I found that IIS would need to be restarted , due to that change of "trust level" in web.config. So for the moment I just removed that line.

 

ALthough I expect that the trust level is important and will need to be set correctly at some stage. Perhaps an overnight restart might be possible.

 

For the first time now I actually can see the chat page live on my website. Hooray!