Thanks a lot.
I've found the ebook-convert binary-file in /opt/calibre.
But when I run this programm (with and without xvfb-run), I get the same error, for which I can't find any solution on the internet
Traceback (most recent call last):
File "site.py", line 55, in main
File "site.py", line 41, in set_qt_plugin_path