@Joel
Meanwhile I disabled the hyphenation via "KoboRoot.tgz" which contains hyph_de.dic and hyph_en.dic with a very high value for hiphenations.
That works fine.
To decide, if I really identified a bug I want to do some further tests. I don't want to report sth. as a bug, when I didn't test it carefully and reproducable for others.
To compare the "Kepub method" with the "kobo_extra.css method":
I prefer the last one, because I don't like the header in kepubs.
With my sentence "I hope I find a way to use that functionality" I meant, that I understood what you and Peter told me. But I just have to check out, if it's really a bug or I made any mistake in the configuration. That's all
Please be sure, I read your posts and advises carefully.
Sometimes it's not so easy to keep everything in mind, because reading and writing in English is quite challenging for me.
Thanks for you patience and all your help until now