Could not load file or assembly 'xxx' or one of its dependencies. An attempt was made to load a program with an incorrect format

Sounds like one part of the project is being built for x86-only while the rest is being built for any CPU/x64. This bit me, too. Are you running an x64 (or uh... IA64)?

Check the project properties and make sure everything is being built for "Any CPU". f you're in Visual Studio, you can check for everything by going to the "x86" or "Any CPU" menu (next to the "Debug"/"Release" menu) on the toolbar at the top of the screen and clicking "Configuration Manager..."

If you get this error while running the site in IIS 7+ on 64bit servers, you may have assemblies that are 32bit and your application pool will have the option "Enable 32-Bit Applications" set to False; Set this to true and restart the site to get it working.

I had this error when trying to use the dreadful Business Objects 4 for .Net SDK.

They ship five BusinessObjects*.dll files, but all of them are 64-bit.

To get my webpage to load, I needed to click on Tools\Options, then change this setting in VS2013: