Based on our experience, these steps can help troubleshooting OpenID login issues.

1. **Are you behind a proxy or firewall?** OpenID requires very long URLs and proxies / firewalls with aggressive settings will sometimes block OpenID accidentally. This has been documented a bunch of times, often in corporate settings. If so, try without the proxy/firewall -- or convince your network admins that OpenID is in fact safe to allow..

2. **Have you modified the default cookie policy in your browser?** OpenID, and our login process, requires cookies to be enabled and match the default settings that browsers ship with. If you have modified the cookie policy in your browser (or even worse, have disabled cookies altogether) this can prevent logins from working.

3. **Have you tried clearing your domain cookies?** Sometimes the cookie information gets corrupted somehow (not quite sure what happens here) but clearing cookies for the domain in question (serverfault.com, superuser.com, stackoverflow.com) has been known to work.

4. **Have you tried in a different browser?** The world is lousy with great browsers, and switching out browsers can quickly tell you if there's something "wonky" with your browser plugins, configuration, etc.

5. **Are you able to log in to other sites that support OpenID?** [Lots of sites](https://www.myopenid.com/directory) now support OpenID [including big ones like Facebook](http://www.readwriteweb.com/archives/the_dam_just_broke_facebook_opens_up_to_openid.php). Does your OpenID work there?

6. **Does your OpenID profile have invalid information in it?** This is *quite* rare, but we have seen instances where profiles with problematic info (like invalid / gibberish email addresses) cause exceptions on our end. If possible, try a new, default OpenID or switch OpenID providers just for testing.

7. **Is your site especially slow to respond?** In order to prevent denial of service attacks, it's common for OpenID libraries to have cutoff times of a few seconds when making HTTP requests. If you're delegating, try putting your delegation on a blank, plain vanilla HTML "hello world" page to remove any processing delays.

8. **Do you use a self-signed SSL certificate?** Because self-signed SSL certificates, if generally accepted, would allow DNS poisoning or other network attacks to enable identity spoofing, we do not support them.

There's also a diagnostic page which might give you more information about any potential problems:

http://test-id.org/OP/Sreg.aspx

After using the above page, check the "recent logs" link to see what is going wrong.

Any other OpenID troubleshooting tips others would like to pass along?