Hmm, it’s possible that you’re running into a bug. Or it’s possible that it’s related to it being the Windows version of Plesk, I’m not sure if that’s supported (I didn’t actually know there was such a thing until now
Just to be super-sure that it’s not something simple – that error will sometimes fix itself if you restart Webmin, which can be done with this command:
/etc/init.d/webmin restart
After doing that, are you able to import your backup?
If not, you may want to file a bug report, using the “Support” link at the top of the page. And then Jamie can work with you regarding some specifics.