I've never had RenX freeze up during an export, sure I've noticed it slows a bit exporting animations but it's never really crashed.
Oh then again it did once when I attempted to export a 500,000 poly model. It couldn't cope with that...
Anyways, strange problem indeed. Are you using editable meshes? (not editable polys etc...) Have you made sure the bones and things are all merged into scene and not referenced? (Shouldn't be a problem if you inported them). Have you made sure the pickbox has AABox ticked under W3D tools? (You'll need export Geometry ticked for that) Anything else you think is unusual?