In the web browser, I typed your site without http://www... it automatically directs me to https://yoursite.com. I'm thinking your site is using https while chat is not? Could this be the factor? Assumed you've created sub-server like this?

Virtualmin> yoursite.com> Create Virtual Server> Sub-server> chat.yoursite.com> Create Server 

Did you want to use Let's Encrypt for the chat.yoursite.com? Here's a screenshot to get an idea of set up.