I have a structure in the overworld that spawns rarely in my modpack that has a dragon egg. However, when I try to obtain the egg using an iron sword, I get this crash:

http://pastebin.com/m8ViG1jw

Honestly I have no idea what happened, because the crash doesn't reference Hardcore Ender Expansion in any way. Can you try it on a pure Forge server? Cauldron isn't supported.

It turned out to be related to Dragon Mounts.

Crash is likely a cauldron issue, unless LAN is not an optimal place to test things relating to servers.

I'll test things on a dedicated server to make sure HEE and Dragon Mounts are not having issues

Left-clicking worked on getting the egg.

Alright, sorry but it's not in my ability to support unofficial server software besides pure Forge. If you find what exactly is causing it, then I might be able to implement a fix, but right now the crash does not point to HEE so there likely isn't anything I can do.

Issue is not related to Cauldron. I replicated it in a vanilla forge server. This might be something the Dragon Mounts dev would need to do though.

I thought it was a conflict between these two mods, but, I think Dragon Mounts might just need to be ported to later versions of forge.