When a player tries to spawn a pet, for example using the command: /pet pig, the chat is spammed with the message "[EchoPet] Failed to spawn pet entity." for about twenty times, followed by "An internal server error occurred."

The console is spammed with this error, which continues until the server is restarted:

Using version 2.3.4-SNAPSHOT-b434

Nevermind, it's because one of my plugins removes entities in the CreatureSpawnEvent, that should be the cause.