For reference, Cube of Force Side 4 is:
Spell Effects can't pass through the barrier.
It is unclear to me what counts as a spell effect. I see multiple potential ways of interpreting this:
No part of a spell, including its targeting, can go through the cube wall. A pebble with Magic Stone cast upon it that is thrown at the cube stops at the barrier as if it hit a wall. A creature with Haste cast upon it treats the barrier as a wall.
No part of a spell, excluding its targeting, can go through the cube wall. A pebble with Magic Stone cast upon it that is thrown at the cube moves through the barrier but loses its magic once it passes through. A creature with Haste cast upon it can move through the barrier but loses Haste. The spells resume after the stone and creature pass back through the wall.
Spells can still target inside the cube, but any projectiles or effects created from outside the cube would not go through. The spell itself must state that it has a projectile or area of effect.