I see you’re hosting your site at GoDaddy. Have a look at thesethreeposts.
A GoDaddy employee writes:
Strange methods. Anyway, you should contact GoDaddy Support and have them try to resolve the issue as it’s probably caused by some rule on the server that you cannot alter rather than by your own Wordpress setup.
PS: For security reasons, consider removing the information about your .htaccess and plug-ins above. When it’s no longer necessary for troubleshooting, there’s no need to have others know which plug-ins you’re using