Error When executing Decision task

[a dolphin]

Hi,
 
I used the SAP delievered decision task TS00008267 in my workflow.
When I click on the execute button in my Inbox, message WL510 appears:
Function executed on the selected 2 entries.
 
Does anyone understand what this means?
 
I ignore this message, proceed with the Approve button, then I get an error:
 
Work item could not be found.
 
When I look at the workflow log, I saw a red light with Action
SWW_WI_RUNTIME_TO_TRANSPARENT. It looks to me that it is trying to execute
the work item twice, I saw Execution Started twice, one with red light and
another with green light.
 
Could anyone advise where should I look for the error?
 
Thank you.
 
SeongPin
 
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