SSL connection error on wp-admin directory

Hi. I deactivated a security plugin (iThemes Security - formerly WP Better Security, I think). And now when I go to my wp-admin directory (or login.php), I get an SSL Connection error.

More specifically:

Unable to make a secure connection to the server. This may be a problem with the server, or it may be requiring a client authentication certificate that you don't have.
Error code: ERR_SSL_PROTOCOL_ERROR

Any thoughts on which this might be happening?

Thanks,
Frank