Help!!! Some unusual FLUX 3D ERROR message

Dear Flux community,

I am modelling a FLUX 3D model in steady state AC magnetic 3D context. The model uses rectangular bar conductor and it has real end-winding connection and a coil conductor is used to represent the winding region in the electrical circuit with input and output defined properly (see the diagram attached). However, when I check the physics before solving, an error saying 'MECDX3_2 => Singular jacobian for element 94095, MECDX3_2 => Unable to compute jacobian' confuses me. This particular volume element is the layer between the infinite box and the actual machine. At first I think it is the coarse mesh of infinite box, so I change it to a very fine mesh, this error is still there. I have also deactivated aided mesh and manually setting the mesh of model but this method also won't help. Does anyone know what has gone wrong? I have been trying to fix this in the past few days but failed and it's really killing me. Thanks for your help.

Yangyu