Spent most of this evening fighting a goofy issue. For whatever reason, the Reversal Icon theme (installed through the Download New Icons menu in Plasma Settings) is causing issues that ultimately lead to Discover and System monitor not opening. When trying to run plasma-discover in the terminal, it spits out the following:
org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: false
adding empty sources model QStandardItemModel(0x55a379b32cc0)
qrc:/qt/qml/org/kde/discover/qml/DiscoverWindow.qml:119:5: QML Shortcut: Shortcut: Only binding to one of multiple key bindings associated with 15. Use 'sequences: [ <key> ]' to bind to all of them.
render glyph failed err=62 face=0x55a379fe7a90, glyph=54
KCrash: Application 'plasma-discover' crashing... crashRecursionCounter = 2
Segmentation fault (core dumped) plasma-discover
That "render glyph failed" appears to be either an issue with the icons itself or the icon cache, based on feedback in the Solus forums, where someone ran into similar issues. Whatever the reason, the issue resolved itself after returning to the Breeze icons.
So, if anyone else runs into similar issues, maybe this will help someone find the root cause of the issue! I didn't run into this on Fedora 43 or Debian 13, so I imagine something in the newest release is the culprit.