I know this problem has been reported before but I haven't found a solid solution so.... :

Invalid backend response.
Data is not JSON.

How to fix this?

I've disabled several plugins and extensions. Funny enough, right after install mm was working.
SInce then I've been working with the articles and categories (fresh install / Joomla import)
Then all of a sudden I wasn't able to assign an image to an article, now I'm stuck!
I've also chmodded everything to 777 but no luck... (I'm on a ~temp.domain at this moment)

Thanks a lot for any helpful replies!

I am having the exact same issue as everyone else on here. The sites were functioning properly for over a year and then all of a sudden, i am getting these errors when attempting to upload.

I am using Godaddy VDS with Plesk on a Windows server. K2 v 2.6.1 / Joomla 1.5.26 AND Joomla! 2.5.4 Stable - I am having the problem on both versions of Joomla on 2 different websites. Both of which were working just fine with no problems.

json support enabled
json version 1.2.1

Got this fixed by installing fcgi/suphp on my server...