I avoid using usb with virtualbox. It's supposed to work, but my VM usually crashes when I try to do it.
Anyway, I've seen that error before, but at the moment, I don't remember what the problem was. If you're running Debian 8 (Jessie) then that's the problem. You need to use a newer version of refracta2usb. There's one in the Testing folder at sourceforge with "sid" in the name that should work. If not, use the beta that's here -
http://distro.ibiblio.org/refracta/file ... a06.tar.gzUnpack the tarball, change to the directory that's created, su to root and run ./snapshot2usb. You might need to install archivemount first.
If you run the version you have with the -d option, you'll get a lot more output in the error log, which is in ~/.refracta/refracta2usb.log. There might be some clues there.
You can ignore the gtk errors.