Log in and Register Pages Not Visible

I run a small home server (Apache)with a few wordpress sites. All have been set up the same way:
1. URL https://www.sitename.com is forwarded to https://sitename.no-ip.info
2. WordPress installation is done using https://sitename.no-ip.info url.

If I upgrade any of the sites above WP 3.1.2 the log in and register pages stop working.

Working Example based on WordPress 3.1.2 https://www.simonfyfe.co.uk If you select Log in or Register the pages work correctly. As does the https://simonfyfe.no-ip.info url.

Not Working Example based on WordPress 3.2.1 https://www.few-fyfe.co.uk If you select Log in or Register the pages don’t work but they do work if you visit directly via few-fyfe.no-ip.info

Any ideas?