If a type can't be loaded for some reason during a call to Module.GetTypes(), ReflectionTypeLoadException will be thrown. Assembly.GetTypes() also throws this because it calls Module.GetTypes().
The Message for this exception is "One or more exceptions have been thrown while loading the types" or "Unable to load one or more of the types in the assembly", which doesn't seem very descriptive. But, the exception actually provides more info that that. Just get the LoaderExceptions property of the ReflectionTypeLoadException instance. That will give an array of the exceptions caught while loading all of the types from the module. If the exceptions are due to an assembly loading problem, see my general debugging advice.