This could be happening for several reasons, but one common one is that the DB File (Database File) is corrupt and needs to be deleted to make room for a new one.

Another sign that this is the issue is that the PBU Processor icon (Pink PBU icon with a gear on it) will be duplicated multiple times in your systems tray (arrow at bottom right corner of screen along with your Helper Icon).  

To resolve this issue simply delete the file located in c:\programdata\photo booth upload\db