Hi, thx for the response.
That is what I assumed - some differences between CR-10 an CR10s - although I hoped you would respond "you assembled it the wrong way". ;-) :-D
Just ordered new GT2-belts - shouldn't be much effort to replace them - it's the easiest way to solve the issue for me.
As well I found some belt end clips on thingiverse (because I don't want to use cable ties).
But again: really good design - thx :-)