Hi there!
As far as I can see on the code of ARPG, I think they are just not compatible at all. The ARPG Core aliases the Sprite_Character for the arpg stuff. The only way I can maybe find out is if you are willing to create a sample project for me, where I can just start the playtest and hit that same error.
With that, I may be able to find a patch. But just looking at the code I don't think I can provide a fix or an answer.
If you are willing to do this, upload that sample project somewhere and send me the link here or through discord:
hakuenstudio