I tried a few tools to check if the pdb and the dll actually matched, and using chkmatch I could see that the GUIDs in the dll being run and the pdb in the obj folder didn't match.
So it turns out that although the dll and pdb in the project's obj folder are a match, the dll that was actually getting copied to the application's destination folder by a post-build event was the old dll from the previous build.
The post-build event was running before that particular project had built, or at least finished building, and was copying in the existing dll from the bin which was subsequently overwritten by the continuing build.
I have resolved the problem by editing the project dependencies for the solution and ensuring that the project with the post-build event is dependent on the project that wasn't loading, and now the pdb loads during debug.
I have the same problem and by reading your solution I still don't understand what you did to solve it. Could you explain more detailed?
Basically my projects were building in the wrong order. When you right click on your Solution in Visual Studio 2012 there's a Project Dependencies option and on the window that opens there's a Build order tab where you can set the order the projects are built in.
I tried adding a pdb file from another folder, downloadng symbols(it started downloading MS symbols, so stopped it), specifying modules to load - None worked. My problem was that I was getting the dll from Nuget and the pdb from the source I had. I copied the dll from the source I had and it worked!