Let me present the context:

1.12.2, Immersive Vehicles Vanity-1.12.2-2.31.1, Immersive Vehicles-1.12.2-22.14.2.

I placed down rail tracks, a locomotive, and 2 trailers: the coal thingy, and a container locomotive trailer.
I spawned the locomotive, then the coalstack trailer, they attached well, then moved forward, and tried to add the container trailer after the coalstack trailer, and I clicked many times the button on HUD to look for trailers while moving slowly, so that when the trailer was in good range, it would attach.

And while I did that, I had a server crash.
Then, I restarted the server, tried to log on, BAM, crash loop.

So I thought it was the chunk that broke, so I managed to TP my player to 0 100 0 before the server crash, so that next time I would log on, I won't crash, because far from the broken chunk. But that wasn't a solution!

It's actually the playerdata that got corrupted:

As seen in the image, I can't even read the playerdata nbt.

So, I deleted my playerdata, and logged back on, it worked fine. But, going back to where I was testing previously actually corrupted the playerdata again.
So any player coming near the area gets corrupted player data.

In the end, I removed all MTS mods, reloaded the area I was testing stuff in without MTS, then brang the MTS mod and packs back, restarted server, and the problem was solved.

crash-2024-06-05_09.08.57-server.txt
crash-2024-06-05_08.58.40-server.txt