Symptoms

When building/compiling projects from Visual Studio with reference to the .NET Framework 4.5 assemblies, error were thrown.
 

Diagnoses

C:Program Files (x86)MSBuild12.0inMicrosoft.Common.CurrentVersion.targets(1696,5): warning MSB3270: There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference "SourceCode.HostClientAPI, Version=4.0.0.0, Culture=neutral, PublicKeyToken=16a2c5aaaa1b130d, processorArchitecture=x86", "AMD64". This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architectures between your project and references, or take a dependency on references with a processor architecture that matches the targeted processor architecture of your project.

 

Resolution

From the K2 blackpearl 4.6.8 release notes (http://help.k2.com/KB001600):-
The K2 Server (Host Server) has a dependency on .NET Framework 4.5 as a result of the change in the way claims functions. Ensure that you have .NET 4.5 installed on the server before upgrading to K2 4.6.8.

A lot of forums did state that when using the .NET Framework 4.5 assemblies, you would need to specify to use x86 or x64 instead of Any CPU.

After specifying to use x86 the issue was resolved