Yeah I had this happen on max 2012, I thought it was fixed by now with a new file output module from autodesk? Either way the lovely Rens has some code that writes a 1 x 1 bitmap to force max to use the right format and the file to write correctly - http://forums.chaosgroup.com/showthread.php?70583-output-passes-to-separate-folders&p=586624#post586624