NextGEN displays an "HTTP Error" message (no other details) when using the Flash-based uploader with NextGEN 1.9.1 and WP 3.3. The problem occurs whether using drag-and-drop or the "Select Files" button. Images can still be uploaded if I "Disable flash upload," but there goes the cool new drag-and-drop.
When the site was running NextGEN 1.8.4 and WP 3.2.1, the Flash uploader worked fine. It's only broken since the upgrade.
And an odd thing: the HTTP error occurs only after the image has been 65% uploaded. (I am testing with images less than 20k, so it's not a size issue.)
The error occurs even if all other plugins are disabled.