It’s a dumb little problem: opening a video on NicoNico has a flash setting dialog pop up asking
res.nimg.jp is requesting permission to store information on your computer.
Requested up to 1MB
Currently used: 83 KB
With “Allow” and “Deny” buttons – neither of which are clickable!. And so this dialog sits in the middle of the video widget refusing to be dismissed and cockblocks the crap out of all your video watching efforts.
Reinstalling Flash as suggested somewhere didn’t help. In the end, I discovered a dumb little workaround: load the video on nicoviewer instead. That is, if your video url was
http://www.nicovideo.jp/watch/sm12345678 then the nicoviewer url would be
http://www.nicoviewer.net/sm12345678. The same dialog should show up on the player there, but this time the buttons are clickable! So I clicked allow, Flash remembers the setting, and videos on NicoNico stopped showing the dialog from then on.
I later also read that an alternative method is to go here where you should be able to see your Flash player settings. Just scroll through the list of sites and look for res.nimg.jp and then adjust the slider up to ‘1mb’, and that should apparently fix it.