Thank you very much for all the informations and comments!
The culprit is definitely ddjvu: https://sourceforge.net/p/djvu/bugs/335/ Fortunately the output in the ppm format is correct, so after converting to png I can use it in my text. Once again thank you very much for you quick reaction to my problem! Best regards Janusz -- , Janusz S. Bien emeryt (emeritus) https://sites.google.com/view/jsbien
