Q: scanimage does work, xsane does not (or vice versa). What can I do?

A1: in most cases this is caused by at least two installations of sane-backends.
Make sure only ony sane-backends version is installed.

A2: may be /usr/local/lib/sane, /usr/lib/sane or /lib/sane is listed in /etc/ld.so.conf
this is not allowed. Remove theses lines from /etc/ld.so.conf and call
ldconfig as root.
