AC/DC model: Matrix has zero on diagonal.

Hi everybody.
I have to calculate the magnetic field around three cables.
The model I built simply includes the three straight conductors and air around them.
A small voltage difference is applied and the Electric Currents module is first run to compute the current flowing in each conductor.

After that I run the Magnetic Felds module, but Comsol returns the following error message:

"Failed to find a solution.
Matrix has zero on diagonal.
Returned solution has not converged."

I'v done many attempts to solve the problem changing the mesh, the way model geometry is built, and even the type of solver, without success.
Could anyone suggest aternative approaches?

Thanks

Massimo