Works fine for me... Are you running this script as a part of an Action?
I created a new sequence (see capture1)
The code can you find in my first call.
When I run the sequence, following message appears:
My conclusion was: something is wrong with script because the pop-up says that the pdf will be overwritten.
The problem still occurs as you can see above. No png is created at all. Acrobat X professional is used, friday update to DC.
I would set the Save To setting in the Action to "Do not save", since you're already doing it with your code...