Hi! I'm justgettiodng into AR and got into this problem with a cause and a solution.
Everything was working (kinda), exporting models from modo and importing to RConverter. USDZ sometimes works, sometimes I got the pink stripped materials (not sure why or how to fix), others times gltf format.
CAUSE (for me at least): I installed Gestaltor to try it out as a gltf editor (looks nice) and then the problem started: every gltf I had changed it's icon to Gestaltor's file and Reality Converter stopped recognizing them either as drag and drop nor import file.
SOLUTION: I uninstalled Gestaltor (maybe not necessary but cleaner anyway), and then getting File Info on a file and selected "Open with" -> Other -> "All apps" from the dropdown -> Reality Converter. Now it worked!
Hope this helps someone.
V