Recently upgraded to 3.5.1
I know it has been asked before, but after 3 days reading here and in google no solution so far for this error.
I am using a twentyeleven child, which i upgraded to twentytwelve child.
If i use twentytwelve the image upload works.
There is no JS errors on the console.
I have tried:
*Turning on Debug
*Disabled all plugins
*Deleted all plugins
*Installed wordpress in another domain from scratch
*left only the child theme from wp-content
*Read trough all the Master list
*All the crazy solutions in several posts (deleting plugins from tinymce, adding lines to wp-config.php jumping on my wooden leg while sacrificing a parrot [this is from my pirate forum])
Finally I found a solution, and it is very simple.
Now the image uploader works with twentyeleven, twentytwelve and my child.
I am making you read all this because of the time i spent finding the solution.
Which I am posting on my site. Not here
Because none of the moderators, or contributors here thought of giving actual help and kept asking users to RTFM, to try the default troubleshooting whatyamacallit, or disabling plugins. Or even worse... Asking the user to create a new question when he was asking the same question with the same problem which went unanswered since the roll down of 3.5
Worse i suffer for all the poor souls that had to start a new child theme from twentytwelve when they had a perfectly working thing on 3.4 and all the time they spent on it.
This post will be probably moderated because of this, but in case they are nice enought to keep this post here is the link ot the answer.