Worklet task instance [XYZ]: Could not acquire the execute lock for Worklet [xyz] [].

I just got this error. The Workflow Monitor showed the worklet in question as being succeeded.

Solution

Stop the worklet explicitly by the use of the context menu.
 It does not seem to be necessary to remove the lock in the Administration Console as suggested by https://kb.informatica.com/solution/5/Pages/18244.aspx