Failed to generate mesh

Hi all,
I am trying to simulate the magnetic field of a permanent magnet. Initially the simulation worked. Later I changed the length of the magnet but an error arose saying

A problem arose when building mesh feature 'Free Tetrahedral 1'.
Failed to generate mesh for domain.
- Domain: 2
Failed to insert point.
- x-coordinate: -6
- y-coordinate: 40
- z-coordinate: 24.8202
Internal error in mesh initialization.
- Detail: Failed to determine cavity.
Error in multiphysics compilation.

I cannot figure out the problem. If I change the lenght to previous value it works. But if I make the magnet longer the same error pops up. Help please