Symptoms

When opening a worklist item link in a notification email or directly in the K2 Worklist Web Part on SharePoint, the error "List item with ID of ... not found".

The settings mentioned in this article http://community.k2.com/t5/K2-Appit/SmartForm-does-not-load-giving-quot-List-item-with-id-of-not/ta-p/81488 were checked and set correctly.

Opening the "Display Form" from directly inside the SharePoint List works without any error.
 

Diagnoses

It seems there were some changes made to the rules of the "Edit Form", this is the form that opens when clicking on a worklist item link.
 

Resolution

It seems the "workflow process ID" was passed into the List View (per parameter) instead of the "SharePoint list item ID" when opening. After setting it to the correct ID in the rules section to ensure that it takes the right input, it's working now.