Thanks for the responses. As it works for others I took the time to test some more and what I have found is that Pose Helper works fine regardless of the options I select (not exhaustive) if the figure is V4.2, but the figure in question is Predatron’s Goblin (only the lores version tested). Still, Pose Helper appears to work unless “Morphs by Joint” is selected—if that is active then Carrara crashes.
As it so happens, the scene was originally created in C7 or C7.1. The subsequent testing was done with a new scene created in C8.5-172 and just the figure being tested. So, at least for this, the issue appears to be Pose Helper’s attempts to manipulate the morphs.
I’m not sure the information is useful, but that is what I’ve been able to determine.
Edited to add: I forgot to respond fully to the points Fenric raised. For what its worth all testing was done with the top of the tree selected. The differentiating factor was the figure (V4.2 vs P3D Goblin) and the option (toggling “Morphs by Joint”). So perhaps it would avoid a crash with “Morphs by Joint” if I wasn’t selecting the top level—I did not test that.