Maybe a late answer, but I had the exact same problem and found a solution for it. So you have to collect all 26 unown and solve those puzzles and go through all secret doors, but you also have to fall in the hole of every secret door (and maybe also of the puzzles). When you've done that, you should talk to the top-right researcher inside the small blue building until he starts repeating his message. The golden plate should now be appearing in the main hall.