I've tried downgrading to 1.9 after using 2.x, with the same results.
I'm using WordPress 3.6, latest PHP and latest NextGen build.
I fixed the non-uploading issue by changing permalink structure, as suggested on this board.
When I use the NextGen Gallery widget with default settings, all of the thumbnails displayed are stacked on top of each other with no spacing. This widget used to be perfect.
I would consider purchasing professional support, but to be honest I've experienced nothing but trouble since the 2.x update.
Any idea how to fix the widget?