Could you please post a screenshot from the cut in the x-y-plane without other cuts shown so that the temperature scale is set to max and mix temperature of this cut?

Thank you!
Or can you send the .sim file to me? (tH3f0rC3@web.de)

Dude, there is no issue with the pattern itself. Without the other sections it looks like the attached picture. Sure, there is the chessboard pattern. But this is just the usual behaviour. It will get better when you refine the mesh, but you will NEVER completely get around it. The trick is to create the smallest mesh giving you results you can live with.

And I'm in a good mood, so I will give you a detailed explanation why this is fact:

There is only temperature value for every cell. That's the value calculated for the cell center, depending on (in this case) the Z-coordinate.
Next there are about 20 more or less evenly sized cells over the solid thickness in my mesh (without the prism layers). And there is a temperature difference of about 70K (without the prism layers). So the temperature difference from one cell to the next in Z-direction is about 3K.
Now the plane section cuts one cell where the cell center is above the section coordinate and next it cuts a cell where the cell center is below the section coordinate. So you will have two different temperature values, although the section is on the same coordinate and you expect the exactly same temperature.
You can achieve this expectation. You just need to create an infinite fine mesh. In reality, this is neither possible nor economic. So why bother about the chessboard pattern when the heat flux, temperature monitors etc. don't change anymore when you refine the mesh and the errors are small enough?

I thought there might be another option I haven't thought of.

I wasn't sure wether to interpret the chessboard pattern as an error in calculation/mesh-size or as a result of the high temperature difference. Now it is clear thanks to your good explanation.

Thanks for your help!

Best Regards,
tH3f0rC3