One or more milling paths for inner contours (cutouts) are missing or incomplete

Hello Community,
I have the problem that I always get the error "One or more milling paths for inner contours (cutouts) are missing or incomplete. How can I fix this? I have changed a few times what unfortunately I could not solve it yet. Instead of only having a thick line on the milling layer, I have now drawn a thin line that results in a closed circle.
What would be the right approach?
Thanks already.
Maxi

The message is a error you can go ahead and order the board with no problems. Our software team will fix the bug.