I've never opened a step assembly, but I'll take a shot at this. Looks like you have component bodies you can open up. Can you open the individual bodies of the assembly, and run import diagnostics on each individual file?
I had the same type of problem before.
The original step file might have geometry issues
I openend each part file with errors and fixed one by one
You can open each part file, do an import diagnosis and fix
It's time consuming work, I don't think there is an easy fix.
Thanks! I was able to open the parts indiviually and run import Diag's. That method just did not occor to me.