I have just redeemed the code in my DO account and it worked on the first attempt. I think they have fixed the issue. To any students out there: try first before reaching out to support, they are busy men/women.
Yep, my code redeemed immediately. They even changed the copy to reflect this: "You can redeem your offer code using the link above, even if you have previously redeemed a different DigitalOcean promotional offer, credit card required for activation"