While troubleshooting another bug-issue of NOF with my site, I temporarily had set a background image for the MasterBorder. Now I removed that setting (as you can see in the screenshot at the right, background image is set to "none") but the image still appears.
Where's the problem? Or, since I assume this is just another bug of NOF12, how can I work around this or force NOF12 to turn back to normal operation?