simulation error flux3D

Hello,

I want to run simulations on a transformer on Flux3D. When I don't draw the skin depth, everything is fine. However, when I add the skin depth and run simulations, I got this message:

I have already checked the geometry, the meshing, the circuit and the physics before running simulation and I didn't get an error. But when I run the simulation and this error appears, it appears also when I check again the physics.

Does anyone know what the problem is ?

Thanks in advance!