"Not Acceptable" Error appearing to users who open protected posts

Hey,

I have a couple of protected posts on my website omahawizard.com

Everything seemed to work fine until lately I started getting complaints from people who try to open the posts using the password I provided them but they get the following error:

http://gyazo.com/8d707737816b917e00bbd13c3f4bcc50

Do you guys have any idea what's causing this?