Karl Heinz, thanks for your effort, but neither an old sane version nor starting xscanimage not from a terminal was the problem. First of all: now it works. I'm not sure, but I did a make clean and a configure without the option --disable-shared and now it's OK. The option was stated in some README to be helpful for debugging, but probably it has some side effects, when the backends and xscanimage are not in the same source package.
Johann
