Hi,
thx all for your response.

it was made about 10 years ago by a college, so no chance to find out how it was made.

the original sketch was quite complex, so it drove me mad to find out what the hell was wrong with circle.

"
Right-click on the grey ellipse.
Pick the Shape Tool.
Press Tab until a point is highlighted off the shape.
Press Delete.
The "Line" is changed to a Shape.

Acorn
"

this was also my solution.
;-)