Context

if you turn the trail spawner into a contraption it crashes the game completely
(if you join again it still crashes)

Crash Report

https://mclo.gs/mvakDUs

This seems like this might actually affect all block entities that end up calling BlockEntity#setChanged when their data is read, see for example #9472 /w Copycats+. It's an issue on 1.20.1 as well. (e.g. aforementioned mod, player skulls /w a skullowner)

To users: if you've been directed to this issue, your world isn't hopelessly lost:
Downgrade to 6.0.6 (including addons if necessary), disassemble the contraption. Now you have two options:

• stay on 6.0.6 for now
• update again, but refrain from using the blocks pointed out as problematic on contraptions.