For some reason, after moving to Illustrator 25.0, all the arcs, circles, curves etc are now made of lines that have distorted, pixellated edges. This is not an anti-aliasing effect - they are irregular bits of white crap in my nice black lines.
Squares or straight lines do not do it. Please see attached file. I have tried setting the brush to basic. In the last version of Illustrator, these were all fine, but something has changed in the upgrade. Help!