Thanks for getting in touch about this and sorry to hear about the error you're receiving. We haven't come across this specific issue before.
Can you try updating to our latest release to see if the issue still exists? You can download it from: http://fastvue.co/tmgreporter/downloadbeta
Simply run the new installer over the top of your existing installation. The installer will pick up your existing settings, so just click next throughout the wizard without making any changes. Once installed, browse to the site and clear the browser cache by hitting ctrl + F5 (cmd + R on Mac).
Alternatively, can you try restarting both the Fastvue Arbiter service on your TMG server, as well as the Fastvue TMG Reporter service on the Fastvue server. Then delete and re-add your TMG Server as a source in Settings | Sources.