Long time ago I did a research of where the Supreme Cube can be used. But today I tried to use it inside of the Otherworld "Heart of destruction Quest" (I was standing on the lever) and I couldn't use it, and when I moved away for the lever I got to use it. So this makes me wonder: In which other places is not possible to use the supreme cube? Thanks in advance.
Aside from many of the rooms before the bosses (where the lever is) as you said, the boss rooms themselves are also blocked from cubing out.
Edit: If you swear at King Tibianus during the Kingsday mini world change, you end up in Thais Jail, from which you cannot either teleport out of there, or log out there.
Basically you cannot use the cube on any No Log Out Zone, which are basically the bosses rooms and other places:
The following are notable exceptions that are not no-logout zones:
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