KDE/Plasma: Ugly mouseover overlay on empty bar

Dear All,

i have an ugly mouseover over a bar i have created at the bottom of the desktop. It is hiding when the mouse is not there and it is not always apearing when i go with the mouse over the bar.
Any idea what this is ?I have tried all default themes, i can’t get rid of it. It is the gray something you can see over the bar.
https://www.opensuse-forum.de/attachment/6559-leiste-png/