Could you provide the scene and steps to reproduce this issue? Currently, we have a known issue: opening Material editor in some scenes causes this error.
I get the same error in max 2018, makes the plugin unusable. Nothing to do with material editor in this case, appears just selecting ProRender in render settings.
![U787IUECG]@`@B[V{MAA%}N.png](/t5/image/serverpage/image-id/15448i67B478045830380F/image-size/large?v=v2&px=999 "U787IUECG]@`@B[V{MAA%}N.png")
![NDMFN@1~41]C5N(R]C}$~P6.png](/t5/image/serverpage/image-id/15449i6D4B2ADD76A35909/image-size/large?v=v2&px=999 "NDMFN@1~41]C5N(R]C}$~P6.png")
