My best guess is you don't have a theme.skin file in your skin folder or if it exists file permissions are not correct on the theme.skin file.
I'm unable to produce the error, but I've added a change revision 403 so that if it fails to get it from the skin folder it will get it from App_Themes/default, that should prevent the error.
There were several fixes made for the virtual path provider for themes about 6 days ago this is the first I've heard of any related errors and there have been hundreds of source code downloads since then. I'm also running the latest build including these changes on this site and on demo.mojoportal.com.
If all else fails I would try rebooting.
Hope it helps,
Joe