Hey GolpeMortal, that issue was driving me crazy, here is the solution, at least on Ubintu 12.10:
on a terminal window type:
gksu gedit /var/www/fog/management/includes/tasks.confirm.include.php
On the .php file look for any occurence of &$tmp and replace it with $tmp (remove the ‘&’), save the file and run the upload again.

Good luck!