When there is an issue with folders not loading like this in the web interface it is usually an issue with the root.cfg of the mail folder or the mailbox.cfg of the mail/subfolder. Or both
One option would be to delete the CFG files. This will cause a loss of email flags ( read, unread, fwd, re, ... ).
To do this you will want to stop the smartermail service
remove the files ( delete or rename )
start the service back up
impersonate the user so that the file is recreated
If you need help with this you can submit a ticket.
I hope this helps.
Technical Support Specialist