Dear Mr. Houston,
our project was on hold for other reasons.
Now that we are back to work on it, I can confirm that your help solved the problem, thank you very much!
The confusion comes from the fact that there are multiple ThreeJS JSON formats and they are all incompatible with each other.
We were not aware of this very specific detail. Since we use the correct ObjectLoader, all issues have gone.
Thanks once again,
Kind Regards,
S.Thomas