It's very annoying, after a while the "reveal desktop" icon (org.kde.discover.desktop) stops working and I don't know how to re-enable it. The error message, after I click it is "Unknown application folder".
Hi, it's a widget.
Maybe try and delete the not working one, and re-add it:
Right click on panel and go to 'add or manage widgets'
Search 'Peek'
Select (you might then need to move it next to the 'K' if that's where you like it)
Widget != taskbar icon, it is a nice workaround, but not a fix. I was thinking maybe if I deleted the KDE config, the defaults would restore a working icon, but I am too lazy to try. Anyone knows?