What could be causing this to happen if it can't be replicated? It's only the nether portal from the overworld to the nether that is causing all the trouble. Is the world corrupted?
Is there any known fix to this known bug yet?
I just updated the post and added the crash report of this world copied from my multiplayer server into a singleplayer. Does that work, or do you still need the server logs?
It also appears that throwing two items throw the portal 1 at a time gives me the "server closed" error and kicks everyone off the server.
I'm not necessarily sure if it was a crash per say. But I and everyone else on the server gets kicked and we get either "Server Closed", "Remote Host Closed The Connection", "Server Disconnected", etc. We've gotten several different reasons that were given to us when we're kicked from server due to guardians entering the nether portal. Minecraft doesn't exit full screen and go back to the client window so I'm not sure how to give you a crash report. Is there another way?
Alright so apparently the nether portal in question was indeed the problem. However I do not know why. I created a nether portal in the overworld within 10 blocks of the portal in question. This new portal was first linked to original portal in the nether and the problem still happened. So I then proceeded to tear down the buggy nether portal in the nether and moved it backward about 2 blocks and now I can throw blocks and let mobs go through the nether portal and I no longer get kicked. It appears that the problem is specific to a certain location in my nether and not the over world. I have a mob collection chamber in the nether for my guardian farm that is only 1x2 blocks big. Maybe it's too small for 1.8.1 and causing them to bug out cause not enough space? However, I don't see why any changes in 1.8.1 would cause this. Maybe a corrupted chunk.