I'm truly sorry for the inconvenience you experienced. Some players reported this problem to me before, but I was not be able to reproduce on my side, thus I could not find out what was the real cause of this. And about the puzzle you are asking, I'm sure you have found and exposed the mask on the Red Giantwood (or Giant Redwood), right? I think I will give you some hints about this first.
- About the "Eye of the night": What can you see in the clear night sky? Use your imagination to know what an "eye" is. It can only be seen at night after the sunset.
- Next, if you have figured out what the "eye" is, try to find out the time it reaches the highest point in the sky. Be fast because it won't last long.
I hope this will help you to solve this puzzle. It is one of my favorite when I designed this game.
P/s: I have seen your post on RPG Maker forum about Aiko's trial (Holy Priest). If you haven't managed to finish it before the invasion, you MUST deal with the invasion first until the battle is done (from Step 96 of the Main Quest if I remember correctly) , then you can go back to the Cathedral and finish the trial.