When attempting to start a Web client we get the error message in the following image:

The client is opened from the zenon server web page

The Web client seems to fail to connect to the Primary Server and gets stuck at this point ("Establishing connection") before failing with the above error message.

What could be causing this? Does the Web client and/or the Web client host computer need to be configured in a certain way?
The server can be connected to with a browser but something seems to be missing for the Web client to connect. We have this working on an other computer. Both appear to be configured in the same way.

We use zenon 8.20.