I don't know why it works right now, but I changed the drawcircle/drawellipse to images.roi.Ellipse with fixed values.
%% nothing changed above
%% define ellipse
% show the image and place a circle to the right ROI
subplot(1,1,1);
imshow(image)
title('Please place circle.');
roi = images.roi.Ellipse(gca,'Center',[200,200],'Semiaxes',[Radius,Radius],'FixedAspectRatio',true,'InteractionsAllowed','translate');
waitfor(msgbox('Please adjust the circle on the original image manually and confirm it here \bfafterwards\rm with ok.','Circle correctly positioned','none',Opt));
roi = addlistener(roi,'ROIMoved',@allevents);
Now it creates a ROI that is fixed in aspect ratio and radius of the circle. You are only allowed to move the circle around your image.