Finally I also got the other html file fixed ... after I stripped it down to about 2 or 3 lines.
In this case, a chapter started with <p>. I once saw something that you also have to use </p> at the end ... there was no </p> so I added it - but still, it didn't work.
So after that I removed both the <p> and the </p> and now it worked. I tried another html file that had reflow problems. I removed all <p> and </p> and the reflow problem was gone.
I really don't understand how such a basic command can cause this trouble.