I assume that the <username> part in the error message actually showed your own user name, right?

Have you checked for common causes like insufficient disk space or a write-protected file?
Does it make a difference when you run The Bat as Administrator?
If you want to try backing up your mail at a different location, see Options | Preferences | System | Button:Advanced.

The free 'Process Monitor' utility by Sysinternals (Microsoft) can monitor all read/write actions, and maybe it can give you a hint of what is going wrong exactly.