I couldn't get my KTN to attach to my reservation using the Iberia website and similarly got errors or system hangs. I then tried the Iberia smartphone app, and had much better luck. By selecting "form to enter the country' and then "other documents (optional)," one of the types of documents is "known traveler ID" and I entered the KTN there. My flight is not for a month and I did not attempt to check in, but it looks as if the KTN has stuck in the reservation along with my passport info, etc.