Rounded corners not working

[weidler]

I am having issues with the displaying of the rounded borders. Where there should be a rounded window corner, there is, but below is a sharp corner in black, as can be seen here:

I did some googling and found out that it might be caused by issues with Aurorae. Is there a way to fix/workaround this issue? (I guess so since i saw many working screenshots :) )

Distro: Debian 9 Stretch

[varlesh]

disable Blur on plasma settings effect

[varlesh]

https://bugs.kde.org/show_bug.cgi?id=395725