Using "html help workshop" I found that both of them had the same version info :-/
Compiled file version 3
Compiled with HHA Version 4.74.8702
So I took a wild guess:
I decompiled the faulty chm ebook,
I created a new project
I reused the TOC file and reimported all the decompiled html files.
I added an index (empty)
I compiled the new project (creates new chm file)
uploaded the chm to my PB and it works excelent now
Yey thx for the help Billi.
You pointed me in the right direction.