Please can you provide more details regarding this problem. When you say you receive an exception, do you mean at the bottom of the Administrator MMC? If so, what is the error code that is returned? Are you seeing any relevent errors in the event logs or programlog?
It could also be worth testing that anti virus scanning is occurring on this WFE. The best way to do this is to upload a copy of the eicar test virus ( http://www.eicar.org/anti_virus_test_file.htm ) via this WFE server. This should be detected by any anti virus scan engine.
i had the websites not enumerate under the manual scan screen. reason was that the websites were renamed different than what sharepoint gave them at time of provisioning. renamed them back to the original names and they showed up again.
i also once again had a problem with the manual scan and the only fix i could find was reinstall forefront. not the best solution, but it was a scheduled downtime and needed to upgrade to sp3 anyways