1) Usually we face this error, whenever a WO is kept idle for some time, and during the same time if the SLA tries to process it , (in order to process it , sla agent needs to acquire the lock) , lock will be lost from the user to the agent .
And when user tries to access the same WO again at later point of time , we face this kind of error .
2) Or during the same time, if other user tries to process the WO parallely , you may face this error .
I'm not sure to get your problem exactly sorry. So you are on the Perform harness of a case (presumebly showing a FlowAction already) and then you click a button to "Edit the Flow Action" ?? What do you mean by that, I don't get it.