I doubt this applies to all of you but I was having this exact issue and found my cause. I was defining my WP_HOME and WP_SITEURL in my wp-config.php file. These did not match what was stored in the database and apparently NextGen is just checking the DB which broke my thumbnails.
So if you’re using these in wp-config.php please change them in the DB or use the the web interface. Even if you are not using the wp-config.php to set these I’d check it just in case that is causing your problem. These can be changed under Settings -> General. If they show greyed out that is because they are being set in wp-config.php.
