Applies to both 1.4.x and 1.5.x versions of OpenCart

http://forum.opencart.com/viewtopic.php?f=2&t=40275

Download the patched "cache.php" file there and drop it into your system/library folder, overwriting the existing.

v1.5.1.2 is just 1.5.1.1 with this patch included. No other files have changed between the two versions