Minecraft 1.16.3 with Dragon Mounts current version 1.0.11 in Forge 34.1.42
(also under Minecraft 1.16.1 with 32.0.108 but not running that any more)
On a Dedicated Server

Logs:

Steps to Reproduce:

On a dedicated server...

1. Have two tame adult dragons. Feed them fish. They enter heart mode.
2. They walk to each other and attempt to breed.
   3. Server Crashes.

On the client game

1. Have two tame adult dragons. Feed them fish. They enter heart mode.
2. They attempt to breed.
3. Game does not crash but no dragon is bred.

Description of issue:

Dedicated server is crashed when any player on the server attempts to breed two dragons.