The bug

If there's no end stone in the end, and the ender dragon is summoned, the server will freeze as soon as the ender dragon lands on top of the end fountain / exit portal.

When this issue occurs, the server will not crash. It will just freeze, and then will eventually be shut down by the watchdog noticing that the server is overworked.

To reproduce

1. Clear the entire end (central island) of end stone (or use the attached world file where that's already done)
2. Summon the ender dragon
3. Remove all end stone that is generated below the portal
4. Build a small platform for you to stand on
5. Switch into survival mode
6. Run /kill @e[type=end_crystal]
7. Press Alt + F3
8. Wait for the dragon to sit on the fountain
   → The server will freeze (you will notice this by the bottom right graph no longer moving)

Video

https://youtu.be/Y41Sbq0OJaY

Original description

We blown up the whole end island with tnt for farms, and when we fight the dragon again, each time the dragon lands above the portal, server infinite loops and stops responsing

the only way to stop is wait for the watchdog to kill the server or manually kills the server.