I've been using the plugin for months without issue, and suddenly I got the "Blank line before XML declaration" malformed and unreadable feed (apparently where something inserts spaces before the opening php; seems to have affected Atom and RSS2 feeds only).
Following all the fix instructions for that problem was fruitless, but disabling plugins one by one reveals the problem was with Nextgen. Restarting the plugin fixed it, but the same problem just reoccurred some hours later (restart fixed again), so I am concerned I have run into a bug. No recent changes I have made to the install seem a likely cause; all I can think of is that I now have a few thousand images in galleries with moderate traffic.
I just upgraded it from .96 to .97 on the off chance this will have an effect, but otherwise it seems inexplicable - I can't reproduce the problem either.