We strongly recommend that you update to PyLith v4.2.0. It fixes a bug related to determining a consistent orientation of the fault surface.
A zero pivot means that the Jacobian matrix for the system of equations has a null space. A common way for this to happen is when there are insufficient Dirichlet boundary conditions to prevent rigid body motion (translation or rotation of the entire domain) or you have multiple faults crossing the entire domain without boundary conditions that constrain rigid body motion of the portion between the two faults.
If you cannot resolve the issue based on the above guidance, then provide a diagram or sketch of the boundary value problem you are trying to solve. For more detail about requesting help, please refer to