On level 38 you don't need to use the portal, I'm not sure how it's even supposed to be used.
Overall very amazing game though! Puzzle design is excellent. Only thing you can improve upon would be fixing glitches. Like if a box is bumping into a wall prevent the room change from happening even if the player is holding the button. How you have it set up the box spazzes everywhere and it makes the parts where you're supposed to ride them a little frustrating sometimes.