Fabi - try setting the Crease Angle in Properties pane for the prop to 180 - smooth polygons doesn't need to be ticked, it doesn't appear to make any difference. Render & check if the issue is there. Then set the crease angle to zero, render & check.

If this is consistent (wim & netherworks have also looked at this) the the problem should show up worst when the crease angle is zero and disappear at 180.

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