Same issue here and no idea how you solved it.
I'm also a returning customer and the url from humblebundle does get redirected to https://paizo.com/paizo/account/redemptions automatically, where my codes aren't recognized. I tried adding a space after every 4th character like in the humblebundle instructions as well as everything else on that page.
Tried Edge, tried Firefox, always "That code is not recognized".
