I am unable to add a new folder to my slideshow wallpapers list.

What I'm Doing:
Right-click Desktop > Configure Desktop and Wallpaper > Wallpaper >
* Select Wallpaper type: Slideshow
* Click [ + Add Folder... ]
* Navigate to the folder I want to add
* Click [ Open ]

What I Expect:
The selected folder gets added to the list of available sources for the wallpaper slideshow.

What Happens:
Nothing. I can click [ Open ] over and over and nothing happens. If I select one of the images and click [ Open ], then the folder selection dialog closes, but nothing changes in the settings, i.e. neither that image nor the directory it's in is added to the list of available wallpapers in settings.

My Setup:
Operating System: Manjaro Linux
KDE Plasma Version: 5.24.3
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.3
Kernel Version: 5.15.28-1-MANJARO (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i7-7600U CPU @ 2.80GHz
Memory: 15.4 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics 620

What could be causing this behavior? How can I successfully add my preferred wallpaper folder to the slideshow pool?

Solution: (Possibly Manjaro/Arch specific?)

The cause of your problem is described in this Manjaro Wiki page and thinking a step further you have the solution that helps you.

• Start qt5ct program
• Change all possible settings Style, Standard dialog, Icon Theme, Dialog button layout, Keyboard scheme to KDE and Breeze or Breeze Dark; Color scheme palette to default and apply the change.
• Perform a new logon to KDE Plasma session
• Result: Plasma looks like Plasma and the dialog window reported is working correctly