SAR calculation in CST microwave studio

I am using CST to simulate microwave heating response of tissue using coax cable. I assign density to materials in order to calculate SAR. But the transient solver will stop at Transient Analysis, Pass 1, Raster=1.000, and never respond.
When I restart CST, I could see the simulation results.

But if I set the density of materials to 0, the simulation runs pretty fast. But I can't calculate SAR.

Anyone know why does this happen?

Thanks!

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