giffgaff have probably flagged your account for failed attempts to purchase, a real pain but done for sound security reasons and which no amount of cache clearing, cookie deleting etc. will cure.
However a quick message to https://giffgaff.com/support/ask/cant_topup_airtime
will alert the agents and they can clear the 'flag.
It looks like an irrelevant link but it captures the data that the agent needs and you can use the further infomation box to explain the problem.