The DefaultOrchardEventBus uses the Event class name interface name combination as the key for caching the interface method used when despatching messages to event classes.

The result can be a mis-match when trying to invoke the method - the interface from the cache and that of the class do not necessarily match - only by name.

Can be fixed by using the interface name plus namespace as the key for caching rather than rely on the class name interface name to be correctly unique.

Orchard\Events\DefaultOrchardEventBus.cs - TryInvokeMethod

//MethodInfo method = interfaceMethodsCache.GetOrAdd(String.Concat(eventHandler.GetType().Name + "" + interfaceType.Name, "_", methodName, "_", String.Join("_", arguments.Keys)), GetMatchingMethod(eventHandler, interfaceType, methodName, arguments));
MethodInfo method = interfaceMethodsCache.GetOrAdd(String.Concat(interfaceType.FullName, "", methodName, "_", String.Join("_", arguments.Keys)), GetMatchingMethod(eventHandler, interfaceType, methodName, arguments));

Will also remove duplicate interface methods that are currently cached.