Every time I try and watch a video, a payment form comes up.
The site will display a sign-up form when you do not yet have a valid subscription and you try to watch a subscriber-only video.
If you know you have a valid subscription, you may be signed-in with a the wrong account. From time to time, people forget what email address they used when they originally subscribed. So instead of signing in with their proper account, they create a new account using a different email address. It is therefore recommended that you sign out and try signing in under a different email address.
Alternatively, your subscription may have naturally expired. You can check the status of your subscription by going to your Account Profile page by clicking your Username at the upper right of the page. Then scroll down to see your status.
If you do not have a subscription, then you can sign up for one. Clicking any video as if to watch it should prompt you to complete your subscription. Once you've completed the process, you may begin watching videos immediately.