This error indicates that something has gone quite wrong in the Gateway. To troubleshoot something like this, you’d want to look at the gateway logs. They are in C:\Program Files\Inductive Automation\FactoryPMI\logs
You’ll see a bunch of files there ending with a timestamp, like FPMIService.log.html.2008-02-08, and one file that is named FPMIService.log.html. This is todays log file. You can open this file up in Internet Explorer, or email it to support@inductiveautomation.com to have us take a look at it.