Our support for Microsoft Forefront Threat Management Gateway is quite new and we’ve just fixed a couple of issues in build 188.8.131.52. In particular, this update fixes the “specified cast invalid error” that occurs when importing the Web Proxy database logs.
Check your version in Help | About. If you are running 184.108.40.206 or above, then you already have this update. If not, make sure you update to your software by selecting Tools | Check for updates.