So using 1.21.10 is good since Survival is on the version which Survival is on too.
Using Simple Voice Chat Mod should not cause any issues.
Since this also only happens on Survival I have concluded a probable cause of the issue:
The Error mentions a falsely encoded packet from "set_entity_data". I could go into more technicalities if you wish but I won't for simplicity.
Almost always the issue is a broken mob in the world. This includes but is not limited to custom NBT from commands, corrupted mobs from older versions or missing attributes. To be exact, there is a broken mod somewhere in the end which got loaded once you entered/got near its chunks. You will need to ask a Staff Member to teleport you to spawn.