Full Error:
Server Error
We are sorry but an error occurred with your request CreditCard/GetEncryptedKey with status 200. Please open a ticket with support and describe what action you were taking and we will get back with you as soon as possible.
For some users logging out and then clearing their cache works.
If that does not help, activate the users account for 24 hours and see if they are able to update the card on the billing page.
if you still can't get it to work after troubleshooting, set up a call for the user and manually enter the Credit Card in Braintree.