The Witness marsh puzzle: Valid solution not accepted? [SPOILERS]

That's because the blue squares are "stuck together". In this state, you need to remove 4 squares in a square shape, not just any 4 squares.

You're wrong in your assumption of why the 1st answer works. What really happens is this :

• Bottom left block at the bottom right (red dots).
• Bottom right block at the bottom left (green dots).
• Top right block at the top right corner (purple dots).
• Square removed (blue dots).

By doing that, you merge the 2 bottom blocks (imagine the middle part with 2 blocks on top of one another). That allows you to remove the 4 blocks in a square pattern (blue dots).