One mistake I found is that in the config-file ther path to mogrify was
/usr/bin/X11/mogrify instead of /usr/bin/mogrify. The "X11" was automatically added during install.
I fixed it. But now mogrify ist looking for a jpg in ..../data/501/thumps.
But there is no SUBDIRECTORY "thumps" in the "501"-directory but instead on the same level:
Therefore - of course - the picture can't be found.
What can I do?