Payment to non BBPS Billers (like Amex, HSBC, etc) are failing on Hdfc bill pay, SBI Unipay, cred and other apps since 5th August. This is likely due to RBI instructions for all cc billers to come onboard the BBPS system.
Almost all cc companies have enabled bbps. Few exceptions currently include Amex, HSBC. Until they enable BBPS, you can pay their cc bills using their upi id or neft. See the thread below for details. Please try with minimum amount first.
See here for payment details:
Thread 'All Bank Credit Card Payment option details'
https://www.technofino.in/community/threads/all-bank-credit-card-payment-option-details.5091/