The download URL for GIMP is pointed to a specific mirror which is no longer hosting GIMP 2.8.4 since 2.8.6 has been released.
Changing the source URL to sourceforge fixes the problem.
(Also, GIMP 2.8.6 is out. ;) )
Thanks for reporting will get the latest version in the catalog soon.
GIMP 2.8.8 has been included and should fix the broken link for 2.8.4