Had the same issue. Kept getting error i2501 constantly and the support ticket only derived me to the i2501 form.

I noticed I had my timezone changed to a different time zone due to combability issues with other programs. Swapped back to my own timezone and system locale since that's the one I have registered for my SE account. Restarted my computer and it accepted the key immediately. Try doing that.

It occurred to me after noticing that the form is asking if I use a VPN, browser updates, date and exact hour of purchases, internet provider, and java.
If this fixed your issue, please do reply so I can be aware that this is indeed the cause and it has to be one of the dumbest customer issues I've seen in managing something on the scale of product keys.