Since I had the same problem with one of my client's website, I had to solved it. So here's my solution that works for me:
Go to /administrator/components/com_virtuemart/classes/ps_main.php.
Add this line of code directly after line 269.
chmod( $exec['param2']， 0644 ）；Yup! That's it. The problem is due to the permission setting of the uploaded files.
Hope this helps!