So, there is annoying mini-quest with color keys and maze to the WC room. I've completed it on 3rd try, since game just plays with you. If you don't have any patience, here is what you do.

Beginning - Go forward; Go right; Door; Go forward; Turn right to the key hole; Use BLUE key; Turn left;
Go forward; Junction - Turn left; Go forward; Turn right to the key hole; Use YELLOW key; Turn left;
Go forward; Junction - Turn right; Go forward until you've hit the wall; Turn left;
Go forward; Junction - Turn right; Go forward on stairs; Go to the key hole; Use GREEN key; Get back to door with EXIT sign;
Go through the door; Turn right; Go to the key hole; Use RED key; Go backward to the WC room in the end of the hall;

If you done all in the order (B, Y, G, R), you'll proceed.