I'm not an expert at MeC, but please read this first before posting any bug report.
Read first: I think that exception is being caused by a mod I have recently installed!
Get a fresh minecraft.jar (Options > Force Update on the launcher) and install your mods AFTER MedievalCraft. The exceptions posted on here only apply on this case.
Exceptions (Client)
* Random exceptions when I have Minecraft Extended installed
-> Cause: MCE is incompatible with many mods, including MedievalCraft.
* java.lang.NoClassDefFoundError: BaseMod
-> Cause: ModLoader is not installed.
* java.lang.NoClassDefFoundError: Packet230ModLoader
-> Cause: ModLoaderMP is not installed.
Exceptions (Server)
* java.lang.NoSuchMethodError: ModLoaderMp.HandleEntityTrackerEntries
-> Cause: Start from a fresh minecraft_server.jar, but install Flan's ModLoaderMP (scroll down to the "SDK's ModLoaderMP" section and get ModLoaderMP Server! Be sure your download states "Mini update to fix the furnace bug" before the links on the topic.)