In class eu.phiwa.dragontravel.core.listeners.EntityListener there is a static import of the RyeDragon entity which is used instead of the IRyeDragon interface.

eu.phiwa.dragontravel.nms.v1_8_R3.RyeDragon

This leads to an exception when enabling

AntiGriefDragons.ofDragonTravel

running a different version than CB/Spigot 1.8.3.

Thanks to Jeppa for the report!