Are you using HTTPS Everywhere? If so, temporarily disable it for Stack Exchange sites — it seems to be breaking the login page. I encountered this same error earlier today.

See "Can't login with Stack Exchange OpenID when HTTPS-Everywhere plugin is active - apparent protocol mismatch" on Meta Stack Overflow

Are you using HTTPS Everywhere? If so, temporarily disable it for Stack Exchange sites — it seems to be breaking the login page. I encountered this same error earlier today.

See "Can't login with Stack Exchange OpenID when HTTPS-Everywhere plugin is active - apparent protocol mismatch" on Meta Stack Overflow

Are you using HTTPS Everywhere? If so, temporarily disable it for Stack Exchange sites — it seems to be breaking the login page. I encountered this same error earlier today.

See "Can't login with Stack Exchange OpenID when HTTPS-Everywhere plugin is active - apparent protocol mismatch" on Meta Stack Overflow

Are you using HTTPS Everywhere? If so, temporarily disable it for Stack Exchange sites — it seems to be breaking the login page. I encountered this same error earlier today.

See "Can't login with Stack Exchange OpenID when HTTPS-Everywhere plugin is active - apparent protocol mismatch" on Meta Stack Overflow