blue line above the menu

Hi there,

just another question:
Since today there was no line above my menu shown.
I work with a transparent header and all I could see was my menu (white font on transparent background) and and a light blue circle in the right corner at the top, which appeared since I worked with the advanced layout slider.

Today there is suddenly a light blue line shown. I checked all options I could have set, but I can’t find it unfortunately: What can I do to remove that?

Many thanks in advance!
Kerstin