This sounds promising, what do you do to fix this error?

First try to rename the .mtl-file so SH3D don't see it.
If you can import the model (it will be all grey) without the .mtl-file, then you have at least located the problem.

We don't know yet if I'm right – but assuming I am:

My best advice is to open the .mtl-file of a model that you know is working, and then compare it with the non-working.
You might spot the error – or you might not.

3D builder is likely to produce the same errors again. Then you have to learn how to wash your mtl-files, or you will be stuck.


Lots of luck!

Cec