[23:11:55] [Server thread/WARN] [minecraft/SynchedEntityData]: defineId called for: class com.github.kay9.dragonmounts.dragon.DragonEgg from class com.github.kay9.dragonmounts.dragon.DragonEgg$TransitionHandler
[23:11:55] [Server thread/ERROR] [minecraft/MinecraftServer]: Encountered an unexpected exception
net.minecraft.ReportedException: Ticking entity

Crashes the server, unable to find / kill the entity

I need the full stacktrace (aka give me the crash log) that warning above the error doesn't look related at all, just from the info I was given.

I can help out as im having the same issue and i know why this isnt working, this issue only occurs when playing on servers, this is because servers read code differently than singleplayer worlds, therefore the mod code would have to be adapted in order to play on a server. the first line of code in the crash report already shows that there is some code missing which causes the server to crash. i will attach my crash report.
This is the link to the report:
https://mclo.gs/97PFk6L

I had the same error, the problem seems that the library "org.codehaus.plexus.util.StringUtils" is probably deprecated. This is function is called only when the dragon has a custom name (Line ref 793 of the TameableDragon.java on master branch).

By removing the dragon's name works fine.

Best regards.

Have a nice coding !

Yup that was exactly it. I was using the wrong library for "StringUtils", should've been using the apache library. Thank you for discovering that! Greatly appreciate it.