Error Code: 406

Oliver,
The 406 looks to be coming from mod_security for some reason. The full error message that's being returned is:

Not Acceptable!

An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

I'm not sure why mod_security is rejecting the request, but that may help you narrow things down. Mod_security, in general, tries to prevent 'bad' requests (malicious, automated, and various other kinds). It may be detecting our tool is automated (not a real browser) and denying it. I tested your page with a few other automated checkers. Most passed, but another failed when trying to get your page as well:
www.internetmarketingninjas.com/header-checker/

With all that said, I just manually checked your page and there were no errors that the Error Checker would have found. You should be good to go, but if you're running into any functional issues, just let us know.

Thanks,
Alex