The crash happened when I teleported next to the ungenerated chunks (like 2 chunks away from the ungenerated parts of the world). Now it's happening every time server is started and I log in again in the same place. From the deobfuscated stacktrace it looks like some infinite recursion involving liquids and playing sounds (weird).

The main thing the datapack does is increasing the lower bound of the world to y=-128. It worked for me for hours before teleporting.

Haven't reproduced it yet, but I'll try. Maybe the stacktrace will be helpful to find the cause. I'm attaching crash logs from RC1 (obfuscated and deobfuscated) and RC3 (obfuscated) and the datapack. Server log has no errors till the crash:

[08:53:01] [Server thread/INFO]: okarkalic joined the game
[08:53:03] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 2177ms or 43 ticks behind
[08:54:17] [Server Watchdog/FATAL]: A single server tick took 60.00 seconds (should be max 0.05)
[08:54:17] [Server Watchdog/FATAL]: Considering it to be crashed, server will forcibly shutdown.
[08:54:17] [Server Watchdog/ERROR]: This crash report has been saved to: /home/mc/18/./crash-reports/crash-2021-11-27 

Edit: looking at the stacktraces I believe this may have been caused by lava and water flowing out of a "wall" between the old chunks and newly generated chunks (when you set the min y of the world to -128 in the datapack, the old chunks still update only to -64, this could be treated as a separate bug). I'm attaching screenshots of these fluids made before I'm getting kicked. The screenshost have been made at ca. y -100