"An attempt was made to load a program with an incorrect format" even when the platforms are the same

If you try to run 32-bit applications on IIS 7 (and/or 64-bit OS machine), you will get the same error. So, from the IIS 7, right click on the applications' application pool and go to "advanced settings" and change "Enable 32-Bit Applications" to "TRUE".

Restart your website and it should work.

Somehow, the Build checkbox in the Configuration Manager had been unchecked for my executable, so it was still running with the old Any CPU build. After I fixed that, Visual Studio complained that it couldn't debug the assembly, but that was fixed with a restart.

In Visual Studio, Right Click your project -> On the left pane click the Build tab,

under Platform Target select x86 (or more generally the architecture to match with the library you are linking to)

I hope this helps someone! :)