Unfortunately, certain network configurations, especially those that use firewalls or proxy servers to protect and segment their inbound and outbound requests, cause problems for some users who try to complete the subscription sign-up form we use with our Email Subscriptions feature. We are working to resolve this issue by being more lenient on our end with how we handle these security requests, but unfortunately there is no short-term solution or settings change we can recommend at this time to minimize the chances you will encounter this problem if you experience it currently.
While we have not ever been able to reliably recreate the problem, we have observations that a few things contribute to its occurrence: overzealous security suite software, a corporate proxy server, or possibly browser plugins like Google Web Accelerator. It can even be caused by a virus or spyware.
If you are able to sign up for a subscription successfully on a different computer — perhaps one on a different corporate or home office network — and the problem does not recur, the network you were using when you first encountered difficult with email signup is likely to have been the cause of the problem.