SGEN: An attempt was made to load an assembly with an incorrect format

My problem was finally solved by this page - http://aplocher.wordpress.com/2012/10/12/sgen-an-attempt-was-made-to-load-an-assembly-with-an-incorrect-format-tfs-2010/

Just in case that page ever disappears in the future, here are the steps involved -

1. In Team Explorer, right click on your Build Definition and choose Open Process File Location
2. Double click on the XAML file that is selected
3. In the designer, select the container called Sequence (this is the top-level container that goes around everything else).
4. In the Arguments list (typically at the bottom), change MSBuildPlatform from Microsoft.TeamFoundation.Build.Workflow.Activities.ToolPlatform.Auto to Microsoft.TeamFoundation.Build.Workflow.Activities.ToolPlatform.X86.
5. Save and close the file.
6. Check the file back in to TFS and try your build again.

I encountered this same issue today. A project would not build on my PC but built fine on other PC's

I eventually fixed it by doing the following:

Right-clicked the project with the error, went into Properties

Selected the Build tab and went to the last option which is "Generate serialization assembly" I set this to Off and the project now builds fine.

The problem disappears after installing the latest Windows SDK which includes the 64Bit version of sgen.exe:

http://msdn.microsoft.com/en-us/windows/desktop/bg162891.aspx

Sometimes (if that one does not help) the older version helps:

http://msdn.microsoft.com/en-us/windows/desktop/hh852363.aspx

For some reason the 64bit version of sgen is not included in the Microsoft Build Tools