I had the same unresolvable error until I started using SANE-1.01.
I appears the prior to 1.01 the snapscan backend contained incorrectly
named routines. (i.e. sane_get_devices in other backends was called
sane_snapscan_get_devices in snapscan.c) This caused the frontend to
simply dispatch to the stub routines in stubs.c.
In fact I was really going crazy trying to figure out why I kept getting
"scanimage: open of device snapscan:/dev/scg0e failed: Invalid argument"
when DEBUG traces clearly show the scanner being added via "add_device".
The problem was not a permission or conf file error as other had suggested.
Gary
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