I've had a weird issue on my site the last few days - a blank page except for "Not acceptable" when trying to add text before a post using the Header & Footer plugin. Once the error occurs I become locked out of the admin area, getting this error on all admin pages, although the public facing pages are still all fine.
Have spent hours trying to resolve it, but to cut a long story short it appears the cause is a cookie. I'm not savvy enough on the technical details to explain why, but anyone else experiencing this problem, if you delete the cookies for your WordPress domain from your browser, you should be able to get back into the admin area again. At least that did the trick for me.