On Right click Configure Desktop and Wallpaper is also transparent.
I created a new user, same problems.
Last full install of Manjaro was on 2018 with Manjaro 18.0. Since then it had regular updates through Pacman.
Only official Manjaro repos we’re used and no Snap, flatpak or arch repositories were ever enabled.
I have not logged events because of my hopeful nature that the problem will go away with the next update.
However, about February time I lost the ability to open System Settings, pop-up menus, and KRunner.
That got fixed when I created a new user.
Later this problem came back for the new user and didn’t recover. Also, creating new users doesn’t help.
But the update over the weekend 3-4 days ago made my workstation useless.
I use this machine at work and for over 3 years Manjaro has been rock solid.
I’ve been having an issue very close to what OP describes with KDE multiple times since last week. I just tried suggestions by @brahma and it didn’t help. No worse, no better. Just no difference. I have no background, no panels, no plasma shell. The only windows I have are 2 apps that happen to be auto-start and they work fine. A black screen, these 2 windows and nothing else. And F12 for Yakuake works.
What is getting me to a working desktop again (for a few days) is:
rename ~/.config/plasma-org.kde.plasma.desktop-appletsrc to something else. Then pacman -Syu plasma-desktop and reboot.
That gets me a desktop with a default background and my desktop icons appear again. I then have to add panels back manually such as: right-click desktop > Add Panel > Default Panel
I have to wait a while, it feels like KDE froze but in a minutes or so I get the default panel and an app launcher again. Then I have to fiddle with system settings getting things back to the way they were.
That works great for a while. First thing I notice is that I can’t switch virtual desktops using the panel as its frozen, application launcher no longer works, can’t right-click on desktop. However, whichever apps are open continue to work fine.
I reboot from here, and I’m back to the black desktop, no icons, no panels, nothing but 2 apps that open by default… they work fine. And I have to repeat this cycle over.
maybe you have some third party applets?
also you try going to system settings/compositor, and uncheck enable on startup, click apply, and try running with this and see if it still happens… when you booted into your desktop just enable it again with alt+shift+f12
try to reinstall these: sudo pacman -S kpackage systemsettings
however there was just released a stable update, so if its already available for you try running it first, if not try to reinstall those 2 packages, then reboot!!
The issue is close to the issue with my laptop, sometime I have to boot my computer 2 times, fist time, on first boot the icons are present on the desktop no task bar no mouse no keyboard incomplete desktop, solution hard reboot, I reboot, the kde progress bar appears and everything is fine, strange.