Known issue. Zombies and skeletons can sometimes spawn with armor, and jetpacks exist as an armored form, which rarely causes a zombie or skeleton to spawn wearing an armored jetpack. However, the mod is hardcoded to assume that only players can wear jetpacks, so it immediately crashes as soon as that zombie or skeleton tries to do anything at all.
A solution is to remove the mod, load the world once, and then re-add the mod. If it's a server, you won't even lose any of the items. In singleplayer, well, they'll mostly be gone I reckon, since your player file is loaded.