To preface: I'm a modder, and so have some more insight into the particular workings of this bug than I would otherwise.
To replicate the bug, just execute the command "/setblock ~ ~ ~ spawner{SpawnData:{id:"minecraft:enderman"}}". The log will begin to be repeatedly filled with an error. This bug also applies to any neutral mob that was affected by the recent changes to anger targeting AI such as Iron Golems and Zombified Piglins.
From looking at the yarn-mapped source code, the error occurs in the code that decodes the data for neutral mobs. It tries to find its anger value and calls a method that gets its anger from the NBT tag, which requires a server-side world object. The method decoding the tag expects only to be run in a server environment, and casts the world to a server-side world. This causes an error when this method is called from within the spawner's rendering code.
Maybe just disable that part of the NBT decoding on the client?
Thank you for your time!
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-189565, which is a report that has not been resolved yet.
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