Symptoms

When a user was attempting to action a task via Smartactions, the following error occurred:

The K2 server could not find the worklist item ProcInstId]__ActivityInstanceDesinationID]. This item may have been actioned by another user.
The full error from the K2 server is 'This process has encountered an error. Contact your administrator for assistance.'.
 

Diagnoses

The management worklist indicates that this task was assigned to the correct user(s) that was actioning the task and the task was in an available/open state.

It turns out the process instance was in an 'Error' state as this workflow had parallel activities in which it was erring on an email event:

"The email address, Null or Empty, does not meet the requirements."

The email event was making use of a Smartobject call to return an email address string to the "To:" field. Execution of this same Smartobject was returning a valid email address. This email error was due to there being a semicolon at the end of this Smartobject call in the "To:" field.
 

Resolution

This semicolon after the Smartobject call in the "To:" field of the email event was removed, the process redeployed, the "Process Migration tool" used to move this process instance to the latest version, and then this error was retried. This resolved the email error and placed the process instance back into an 'Active' state which allowed the user to action this task via Smartactions successfully.

http://community.k2.com/t5/K2-blackpearl/Process-Version-Migration-Utility/ba-p/73689

* As this tool was not developed by K2, it is not supported by K2 Support. Please thoroughly test before implementing.