There are several configuration/setup problems that can result in an inability to launch a HTML Access (Blast) connection successfully.

Problem Solution
Browser is not HTML5 compliant Check that the browser version is one cited in the requirements.
Pop-up blocker enabled The browser's pop-up blocker could prevent opening the new window for a HTML Access connection. Make sure that the user disables the pop-up blocker for the Desktop Portal.
Windows firewall disabled Make sure that the Windows Firewall is installed and running on the user's desktop. A disabled Windows Firewall will result in errors reported in the HTML Access logs.