406: :Not Acceptable

odd question, what browser are you using?

and did you already check out this thread?

can you make the change through a file manager with your hosting company or ftp/ssh?

do you know if mod security is enabled on your server?

what happens if instead of accessing /wp-admin/ to ty to hit /wp-login.php directly?

that old thread says it was a related to host security. have you happened to check with your host?

i think you should call hostpapa, other people have had to contact them for mod_security loosening as well, even though it was a while ago