I'm not sure what's causing this, hence the cross-post.

I have an assembly, let's call it Classes.dll, which contains a few classes
that are used by other assemblies.

I have a WinForms program with a UI, business, and data layer (i.e., an .EXE
and two supporting .DLLs). The business layer needs to send a message to a
message queue. It uses one of the classes from Classes.dll as the body of
the message. Code looks like this (my very small business layer is currently
just a static class):

public static void SubmitJob(PackageEntity package, ScheduleEntry schedule)
{
   // some code
   AddImmediateQueueEntry(instanceId);
   // more code
}

private static int AddImmediateQueueEntry(Guid instanceId)
{
   QueueActivityRequest addRequest = QueueActivityRequest.CreateAddRequest(
       instanceId,
       _taskExecutorId,
       DateTime.Now,
       _applicationId,
       false);
   // more code
}

QueueActivityRequest is the class in question. This line is causing the
TypeLoadException error mentioned in the subject. Interestingly, when the
IDE breaks, it breaks on the calling line (AddImmediateQueueEntry above) as
opposed to the line where the object is being set, EVEN IF I go into Debug |
Exceptions and tell the debugger to break when CLR errors are thrown. I even
moved the code out of the AddImmediateQueueEntry function and into SubmitJob
and at that point the code broke at the call to SubmitJob!

I have cleaned the project. I have rebooted. I have removed the reference to
the assembly and re-added it. I have made sure that the assembly is copied
locally. This error continues to happen.

As a test I created a simple console app, referenced Classes.dll, and
instantiated a QueueActivityRequest object (using the same static function I
used above) and it worked perfectly.

Any idea why my business layer hates this class? Karma, perhaps? Any
suggestions are welcome.