CPacketEvents are now async events, they are now fired from the actual Netty thread of the packet's context, this fixes some possible bugs (like items being invisible when dropped on certain versions).
This may break some plugins using this Core, if that's the case contact their author linking to this update notes, you will know this is the cause as the console error may talk about "asynchronous events" or "CPacket"s. If I'm the author of said plugin just message me and I will fix it.