https://pastebin.com/YtwK1yiW

It seems to require the player be marked, there be more then one assassin, and they use Waystone's Warpstone to teleport interdimensionally.

Still unable to reproduce it under those conditions, but I've attempted a fix based on the crashlog, hopefully it works!
You can try it out in the build from here: https://jenkins.girafi.dk/job/Team%20Metallurgy%20Mods/job/Atum%202/job/Atum%202%201.15.2%20Jenkins/

+1 to this. If I have the Marked effect and elytra/rocket away after a pyramid, the server crashes. https://pastebin.com/0KzLu5Mz
https://pastebin.com/JhG0VG1E

+1 to this. If I have the Marked effect and elytra/rocket away after a pyramid, the server crashes. https://pastebin.com/0KzLu5Mz
https://pastebin.com/JhG0VG1E

Could you try updating Atum 2 to the latest version, and see if it still happens?

I'm closing this, as there have been no response for 14 days.
The attempted fix is on CurseForge in 2.1.10 or newer.
If you encounter the issue again in that version, please open a new issue!