Seems to be due to an entity near you. I'd suggest trying to kill all entities or just regenerating the nether world.
I should have written this a lot better, so basically what happened is that i go into the portal from the overworld into the nether, i walk a bit and then i get to a fortress ( i have already been there ) and right next to the fortress i have another portal cause i am never able to remember the way back to the original portal. When i enter that second portal from the nether to the overworld i remember being in the overworld for a few a seconds working fine, but almost right after i got this error and now i can't get into the world at all but everyone else can. Something else that is pretty weird is that the error kinda changes as 50% of the time i get Internal Server Error and the other 50% i get An existing connection was forcibly closed by the remote host. So the nether probably doesn't have a lot to do with this. And my friend that is able to play went to the portal killed off a few entities surrounding it (wasn't a lot) but it still didn't work.
