Strange problem with Xsane

I have a Canon USB flatbed scanner that is supported by the 'plustek' backend in SANE. If I use the XSane GUI program to scan, or specifically, use the preview function—the damn thing sits there forever and eventually begins returning all black pixels in the preview window. The scanner, during this whole procedure, doesn't make a sound or do anything. Now, if I use the 'scanimage' command line program, it works correctly. The scanner is detected by both frontends, so it is not a USB permission problem.

Any ideas?