Strange Problem with Figures imported from Blender / Poser

I'm currently working on a game demo that features a character that started life in Poser 5, was imported and re-rigged in Blender, and exported to Unity. In Blender everything -- The mesh, rig, and animation -- seems fine, but when I import the figure into Unity and place it in a scene, the figure is distorted, as shown in this image, though sometimes it's a leg that gets distorted... It varies on different import attempts:


In addition, when I set the rig up to be Humanoid, I get a stream of multiple errors in the console, all saying:

Next vertex not found in CheckWinding()
UnityEngine.GUIUtility ProcessEvent(Int32, IntPtr)

I have no idea how to proceed to troubleshoot this issue. It only happens with meshes that were exported from Poser, so I'm wondering if there's something that about the meshes that either Blender or Unity doesn't like. Has anyone else encountered a problem like this? Any ideas what I should do...?

Here's my system config:
Windows 10 Home
Poser 11
Blender 2.8
Unity 2019.2.6f1

Thanks ahead.

Dave