@ Hans Passat - Thank you for the hint on Global Flags.
Here is an excellent blog post on using the tools. The gflags.exe program referred to by most of the Google references to "Show Loader Stacks" is part of the Windows Device Driver Development Kit (WinDDK) and it manages the Registry settings mentioned in that post for you.
The output from running with "Show Loader Stacks" identified that a symbol that should have been in my unmanaged DLL was not in the DLL I was using.
It turns out that I was building the wrong version of the DLL. This was due to my poor understanding of the build process (too many versions built too many different ways).