I have to report a very unpleasant situation happened this morning. At 6 (sunday), I was about to use a coupon to enter the ATH lounge but suddenly the service stopped working. The Lounge Agent said she can't see my profile. The phone agent did not help me. I tried to call again, the phone agent this time asked me to talk with the lounge agent, they had a brief convo in greek and finally they let us in (some 20 minutes from the start of this thing). I tried to get in the website/app in all the possible ways with different browsers and devices but "Scheduled maintainance" was always prompting. Lesson learned: before a flight, print or take a screenshot of the coupons, you never know what might happen!