edit the gimp rc files to choose the correct device
I am using Fedora Core 4 and having a similar problem with gimp-2.2.7-4.
The command "scanimage" succeeds but gimp fails. After using strace
to see what scanimage was doing, I noticed that gimp was trying to
access a different usb device. Gimp used epson:libusb:001:005 whereas
sane-find-scanner finds it at libusb:001:007, so I greped ~/.gimp-2.2
for 005 and found it in files "menurc" and "pluginrc". I edited those
while gimp was not running, and the next time I ran gimp worked like a
charm. Obviously this is a work around and not a fix.
If this is not your problem, try becomming root and changing the
permissions of your usb devices. "chmod a+rw /proc/bus/usb"