Shimapan

As expected, turning off Antispam, Cache and so on just was a waste of time, it didn’t change anything. (How should it, after all, as they’re all completely unrelated to media upload.)
What did you change in the media uploader from 3.9.0 to 3.9.1 that made things break?
As I said, I had exactly the same plugins with exactly the same theme in 3.9.0, yet the instant media upload worked just fine.

There, it’s not just me who’s having this problem.
Plugins and theme are exactly the same I had when using 3.9.0, where the instant media upload worked fine.
None of my plugins touches the media uploader in any way (stuff like Antispam Bee, BackWPup and W3 Total Cache… and yes, I’ve cleared all caches, repeatedly).
It has to be some of the changes that you did to the media uploader from 3.9.0 to 3.9.1 which broke it, so that the instant media upload doesn’t work anymore.