Exact same has happened to myself. I’m Silver and have CE return booking towards the end of July. It’s been booked for several months and row 1 has been blocked out as per normal. A few days ago row one appeared to open up for selection however when trying to move into it I get an error message.
As mentioned above, row 1 normally does open up for selection for Silver, Bronze & Blue at T -72 hours. I find if you’re quick off the mark at exactly T-72 you can usually find free seats in row 1.