I have never had an issue before but I am now not unable to import a Rhino 3dm file, STEP from Rhino and obj also have issues.. STL is fine.. Even older Rhino files will not go into Simlab, not even older versions so no sure if something new is effecting older SL software.
I didn't find the right solution from the internet.
This is another general question better suited for the main forum here... https://discourse.mcneel.com/
I'm not sure what version of Rhino you are using but if a file format does not import into your other software but used to in older versions of Rhino, I'd suggest using SaveAs and choosing an older 3dm format such as Rhino 5 3dm. Regarding Step files, you'll have several options when exporting, I like 214 but I don't use SL myself.