"Bad http response" sounds weird. It is like ROS received unexpected reply.
Fortunately, this can be debugged very easily with packet sniffer. If you don't want to dig into that, you can just download the package manually:
http://upgrade.mikrotik.com/routeros/6. ... .43.12.npk
or straight in ROS:
[admin@mikrotik] > /tool fetch url="http://upgrade.mikrotik.com/routeros/6.43.11/routeros-x86-6.43.11.npk"
If you see same size, you know it is right. However if you see different size (I assume there will be something really small) then it most likely means your HTTP transmission was altered by some middleman... Since it is just HTTP, there is no protection of transmission. Personally, I am really curious how long will it take till someone finds a way to hack RouterOS by spoofing downloaded packages with something else...
Anyway, if you download your file and it is not correct size, download it to your computer and have a look inside. Maybe it is some captive-portal http page or something else...