Using 0.13.4: http://hastebin.com/gajokahila.pas

Related to: http://www.minecraftforum.net/topic/1753435-1516-just-another-spawner-jas-v0130/page__st__880#entry28422956

Occurs both without and with MCPC+

Test Pack Please Ignore is the ftb subreddit community pack, located here: http://www.reddit.com/r/TestPackPleaseIgnore
(FTB code: MegaSharkPunch )

note my original repsponse to this issue: http://www.minecraftforum.net/topic/1753435-1516-just-another-spawner-jas-v0130/page__st__880#entry28424786

The testing environment was very useful. Didn't know about FTB codes.

It appears the offending class is chylex.hee.entity.EntityMobMinion from HardcoreEnderExpansion.

Removing HardcoreEnderExpansion allowed the server to proceed normally.

In the interest of continuing to use the mod pack, what should be my next step?

I can report the issue to HardCoreEnderExpansion though I'm not familiar enough with MC modding to determine who should fix the conflict, or what exactly it means (As you explained pretty thoroughly what causes it and what it is)

You'll need to report it to HEE, there is nothing I can do on my end.

I took it up with Chylex, and he's uploaded some kind of temporary fix but had concerns going forward about JAS spawning custom mobs: https://twitter.com/chylexMC / https://twitter.com/chylexMC/status/428645671764520960

Its not spawning it. Its creating an instance to determine its type (if any); which can only be determined by an instance. Entities that are not valid should be not be being registered in production code.

I'd be happy to hear out his concerns. But being honest, this feels more like he's placing landmines in the front yard and blaming others for walking on them.