I had this same problem, was really annoying. I read the reviews and saw loads of recommendations to call them up, so I did, they sorted it out withint 30 seconds of getting through to customer services. They said that my previous payment was very small in comparison to the payment I was trying to make, therefore there was something on the system blocking me from making the payment, so they removed the block. I clicked make payment while still on the phone to them (just in case), logged in to process the payment, went through successfully. Hope this helps.
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