Minecraft starts crashing upon re-entering the world, and it crashes with a few different messages, such as Exception Ticking World, Exception in Server Tick.

Not 100% sure what is causing this, but I think it may have something to do with entering and exiting the nether, then leaving the world. I had left my game after leaving the nether, then struggled to reopen the world. The world would start to open, reach the "Downloading Terrain" screen, then crash. After a while, it did manage to open the world, and I could move for a short period of time, and then the screen would flash black then crash.

It's somewhat hard to pinpoint what started the crashing, but I would hazard a guess that it relates to the Nether portal.

Hope this helps somehow!