On my Mac, using Firefox, I've had this error ever since I got my trade card and attempted to log into my account. I have to log in using Safari for the Halfords site and have no issues then.
I normally use Firefox, but I've also tried Edge, Chrome and Safari....................no good with any of them. And now I'm locked out of the account. And having rung the customer services dept they don't reckon to have any issues with the website :/