Question Contains Spoilers:
At a certain point near the start of the science club event chain, just before you start getting brainwashed, it says you won't be able to teleport to the other world. Sure enough, the next time I tried, Serena simply said she didn't feel like going in the basement.
I assumed that meant I could NEVER go back EVER AGAIN. It's been many days (in-game time), and I haven't been back. I've been keeping this whole play-through on separate saves and holding on to the old saves, so that I could try the "not science club route" later.
Finally, I bought a new painting to cover the hole in the wall, and when I went to place it over the hole, I accidentally walked into the basement. So, I tried the teleporter, and it worked fine.
Is my return to the other world a glitch, or am I an idiot, and it was only a temporary restriction?
(Which is my polite way of saying, that was not very clear, and also, AAAARRGH!)