We are not aware of any difficulties on the site that would cause such an issue with the popup to add a credit card to your account. I tested this functionality in my own account and was unable to replicate any issues with adding a new card.
I'd recommend that you first try clearing your browser cache and then return to the site to try again. If the difficulty persists, please file a support case from under the Help tab of the site letting us know what occurs for you when you try with a clear cache as well as your Operating System and Browser information so we can look into it further.
I have tested the site and had no difficulty adding a credit card using either Chrome or Firefox.
It is possible that there is a caching issue within your browser. Please try clearing your browser cache prior to returning to the site and then try again. If you continue to have trouble, please see if the behavior differs in another browser and then let us know the results you encounter in a support case so we can look into it further.
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