get 'specified cast is invalid' when trying to attach file

I purchased Virtual Restore 2.x about 6 weeks ago and am just now testing its use. At the same time, I purchased an upgrade to my SQL Backup 5.1 - I just completed installing v 6.4.0.56. In order to test VR, I did a backup of a small database with new SQL Backup version and then tried to use VR wizard to attach it. When I try to attach the backup file (I have tried an sqb compressed non-encrypted file plus a native sql 2000 file), I get the message 'Specified cast is not valid'. I can't find any information related to this message in the support site although I figure it must be something simple. Can you share any info on this?

Let me know if you need any additional info.

Thanks!