Threw an error from 11 to Mubox. Figure is Nova with 2 levels subdivision. Might be a Mudbox thing though, honestly I don't think I've ever morphed in Mudbox without using the PML exporter. I've been using it so long that my memory is vague on that. I'll have to test that particular with a regular FBM without PML exporter. In any case I'll test via Blender 2.92 and report back. Here's the test morph imported into Poser from Mudbox as a prop. Here's the Python Error message.

I do have one question however, you didn't mention whether 'Include Existing Groups in Polygon Groups' was requirement during initial export. In this instance all I checked was 'as Morph Target' as you stated in your instructions...