Symptoms

When deploying any workflow from K2 designer we saw the following:

System.ArgumentNullException: Value cannot be null.
Parameter name: path1
at System.IO.Path.Combine(String path1, String path2)
at SourceCode.Workflow.Authoring.Process.EnsureRequiredReferences()
at SourceCode.Workflow.Authoring.Process..ctor()
at SourceCode.WebDesigner.Authoring.Forms.GenerateProcess.Generate(String LocalHostConnection, String UserName, Int32 ProcessID, String InputData, String ProcessXML, String SqlConnectionString, String HostServerConnectionString)
 

Diagnoses

While the precise cause here is unknown we do know that at some point it was decided to upgrade to .NET 4.6 which is incompatible with BP 4.6.8 and after that was realized the decision was made to revert back to .NET 4.5. After that, workflows weren't deploying.
 

Resolution

Running repairs against both the BlackPearl and SmartForms resolved this issue.