What happens if you download the file you shared here and try importing it into the other program What happens if you apply some textures to it before importing it?
Does Plan7Architect have a user forum? If they do it might be worth asking there since this seems to be a problem at that end.
Plan7Architect does have a form but since I was able to successfully import any other SketchUp models successfully they suspect something is missing or incorrectly created and suggested asking in this forum.
Didn’t think about any textures, I’ll add some and see if anything changes.