.net – sgen.exe build error “Unable to load one or more of the requested types”

I am building a solution which contains several solutions. All but one of them build without any problems. The problematic one emits the following message:

Error 2 Unable to load one or more of the requested types. Retrieve
the LoaderExceptions property for more
information. J:\MyPlugins\MyPlugin1\SGEN MyPlugin1

I turned on the verbose output in Visual Studio and I got the
following: C:\Program Files (x86)\Microsoft
SDKs\Windows\v8.0A\bin\NETFX 4.0 Tools\sgen.exe /assembly:

I checked the output with FusionLog and everything loaded correctly. This turned out to be a red herring. I decided to run the build again with the verbosity turned up. I got the following:

Microsoft (R) Xml Serialization support utility [Microsoft (R) .NET
Framework, Version 4.0.30319.17929] Copyright (C) Microsoft
Corporation. All rights reserved. If you would like more help,
please type "sgen /?". SGEN : error : Unable to load one or more of
the requested types. Retrieve the LoaderExceptions property for more
information. The command exited with code 1. Done executing task
"SGen" — FAILED.

C:\Program Files (x86)\Microsoft SDKs\Windows\v8.0A\bin\NETFX 4.0
Tools\sgen.exe /assembly:":\MyPlugins\MyPlugin1.dll" /proxytypes
/reference:"J:\MyPlugins\MyPlugin1\packages\FluentValidation.3.4.6.0\lib\Net40\FluentValidation.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\mscorlib.dll"
/reference:"J:\MyPlugins\MyPlugin1\Libraries\Core\bin\Release\Core.dll"
/reference:"J:\MyPlugins\MyPlugin1\Libraries\Services\bin\Release\Services.dll"
/reference:"J:\MyPlugins\MyPlugin1\Presentation\Web.Framework\bin\Release\Web.Framework.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\System.Core.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\System.Data.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\System.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\System.EnterpriseServices.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\System.Web.dll"
/reference:"C:\Program Files (x86)\Microsoft ASP.NET\ASP.NET MVC
4\Assemblies\System.Web.Mvc.dll" /reference:"C:\Program Files
(x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\System.Web.Services.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework.NETFramework\v4.0\System.Xml.dll"

What is causing this error and how do I fix it short of turning off SGEN? FYI, I didn't write the code.