I encountered this issue with local installation of WP v3.6, OpenID v3.3.4. I already installed both the OpenID and XRDS-Simple plugins, and configured OpenID settings under both Users > Your OpenIDs and Settings > OpenID. I did not modify the wp-config.php file in any way, and I set the OpenID URL in “Your OpenIDs” to point to my localhost WP directory.
Do I need to install JanRain OpenID-Enabled or do I just need to tweak my OpenID configuration within WP?