This; https://youtu.be/bPojsftKZQQ Youtube video explains how to reproduce the specific item in a spawner crash.

The problem this gives is due to the ease of player being able to bring in hacked items they can bring in this Painting type of egg and place it in spawners. This will lead to everyone joining and looking at the spawners to crash.

Reproduce in text

Give yourself a spawner with this command: /give "playername" mob_spawner "ex 64"

Give yourself an painting spawn egg: /give "playername" spawn_egg 64 83

place the mob spawner around or in front of a player. Now right klick the painting spawn egg into that spawner. This will make the player looking @ the spawners crash.

A little explenation on how to get away from it

The way that the "painting crash" works is: basically if a mob spawner is illegally placed, and "activated" with illegal painting spawners, this will cause a crash every time the painting spawns in loaded chunks (in purevanilla 4 chunks), however if you enter the server/world in 3rd person camera prespective you will load the chunks much slower and you will have much more time to walk away from the spawner.