Symptoms

When trying to configure the Start activity, the following error pops up:
Value cannot be null
Parameter name: registeredPropertyWizard

Resolution

This was related to the issue described in this KB article http://help.k2.com/kb001382 with the addition of the steps below:
* Open the .kprx in a xml editor
* Search for “SourceCode.Workflow.Design.Sharepoint15.ProcessDefinition.SPProcessPropertyWizardDefinition”
* Search for the last node in the “PropertyWizardDefinition” node.
* Ensure that the “K2 for SharePoint Process Property Wizard” node exists.
* If the node does not exist, add it manually.

Reconfiguring K2 for SharePoint on the client machine should help prevent this error from happening