KDE Plasma taskbar context menu transparency issues

If I right click on an icon in taskbar, the context menu is completely transparent - almost invisible. And the funny thing is that I couldn’t even take a screenshot because Ksnapshot wouldn’t show up so I had to take photo with my phone.
I couldn’t find a setting for this anywhere so if anyone has an idea how to make it non-transparent let me know. It’s the same problem with any theme I try.

http://xpam.pl/random/transparencybug.jpg