That seems odd - why would rectangles cause slivers?
Flags copper sliver and solder mask sliver areas. This compares the top solder mask layer against the top electrical layer and the bottom solder mask layer against the bottom electrical layer as defined in the CAM documents.
- Minimum Copper—Specifies the minimum value for copper slivers. This flags slivers with less area than this value.
- Minimum Mask—Specifies the minimum value for solder mask slivers. This flags the slivers with a width less than this value, checking the top and bottom solder mask layers if they are visible.
You might try changing your sliver threshold under the <Setup> button for Fabrication.
I tried reducing the size of the flags in the setup and it only reduced the number of errors a very little bit.
I think there is a setting that rounds the corners of those cutouts but I do not know what or where it is.
If you go to outline mode ("O") you can see exactly how PADS is drawing the cutout. For a rectangular cutout it will be a 90 degree corner so there should not be a sliver. Issueing "O" again will get you back out of outline mode.
You might try adding small (10 mil) miters on the corners of the keepout to see if that changes anything. If you want to get fancy you can then change the miters to arcs and see if that improves things. I had to manually convert the miter to an arc with radius, start angle, and end angle; the auto arc function failed so it is tedious. Cut and paste is handy after you get one converted.
Or is this just a bug in the DFF?