The web isolation client application loads its resources and the web socket connects to a TIE gateway but the page remains blank.
A potential cause of this issue is when the remote browser hangs loading a page due to resource blocks in policy.
Review the session logs for the site under Reports > Activity Logs to determine what resources are blocked and by which rule. Add these to an Isolate rule above the Block rule.
You may need to do this multiple times if one resource requests another and so on.