Multiworlds issue, Can't teleport back because of error.

I realy need some help with multiworlds. I'm trying to run 3 worlds: "world1", world1 nether and "Flatgrass" with flatgrass having a seperate inventory and also having multiworld permissions to allow people on the flatgrass map to spawn items, but because of multi inv mod they can't tak ethem back into the legit world. (Its gonna be a freebuild world). My problem is, I can set this all up but whenever teleport from one world to another and then try to get back to the original world, using multiverse and/or simplewarp I get the error "an internal error occured" or something like that and I become trapped and I am forced to restore my server. I have tried many things and spent over 2 hours trying to sort this out :/

Can somone, if you have a simmilar set up to how I want mine could you explain how you have done it or help me with what you think is causing the problem.