eagle wheel file
So, thanks to Old Nick and QELEC.
The Cadsoft support provided a more precise GERBER export function.
I will post it in here. The text has to be copied into eagle.def from the bin folder.
With these CAM-jobs "GERBER_RS274X_25" and "GERBER_RS274X_26" the effects, described above are minimized (0.04 microns).
As a design guideline:
You should use WIRES to define the boarders of areas and Polygons to fill it (course)
Then, polygons with higher width can be used.
Anyway, the wires are a bit misplaced (difference between .brd and GERBER).
You need to use the "more precise" CAM jobs GERBER_RS274X_25/26.
I still have problems with my layout design. My GND areas do not connect GND areas from library elements, due to the specified minimum copper_to_copper distance in DRC. So I had to set it to 0mil, which is a workaround but not good style.
...elektr0