My Wifi Key has also 20 characters 🙂
Now, it seems to me that there is a small error in the interface. If it present to you, to enter the Wifi key, you dont see to the left, as you expect that you are at the beginning of the enter field. But I found sometimes staying letters from the regenerated keys before my password. Best way, connect a keyboard to the game computer. I use a thin bluetooth keyboard with integrated touchpad. Also you can now enter the password directly in normal letters from your keyboard, without use of the virtual one. And before all, look if there is no staying trash on the invisible left of the input field.