SUMMARY Using X11 STEPS TO REPRODUCE 1. Change scaling e.g. from 2.0 to 1.0, hit apply. 2. Go to Fonts and change DPI OBSERVED RESULT System settings crashes EXPECTED RESULT Settings does not crash SOFTWARE/OS VERSIONS Linux/KDE Plasma: (available in About System) KDE Plasma Version: 5.17.2 KDE Frameworks Version: 5.62.0 Qt Version: 5.12.4 ADDITIONAL INFORMATION I understand that when you change scaling this also changes font DPI so manually changing it right after changing scaling is stepping on something. The reason I changed DPI after scaling is becuase I wanted a different DPI then the one it defaults to and wanted it to take effect after my next restart.
If an application crashes we need a backtrace to do anything. You should see a sad face icon in the system tray after a crash occurs.
Created attachment 123666 [details] trace I'm on a different machine with another distro, but can reproduce the error, the icon you described does not appear but there was a trace printed in journal. let me know if this is sufficient, I can recreate in my other machine later this weekend.
Relevant part of the backtrace: Process 1795 (systemsettings5) of user 1000 dumped core. Stack trace of thread 1795: #0 0x00007fb6613cea0e _ZNK7QWindow8isActiveEv (libQt5Gui.so.5) #1 0x00007fb65087e454 n/a (org.kde.desktop.so) #2 0x00007fb660fbeb70 _ZN11QMetaObject8activateEP7QObjectiiPPv (libQt5Core.so.5) #3 0x00007fb650880ca7 n/a (org.kde.desktop.so) #4 0x00007fb660fbeb70 _ZN11QMetaObject8activateEP7QObjectiiPPv (libQt5Core.so.5) #5 0x00007fb6613bcb66 _ZN15QGuiApplication14paletteChangedERK8QPalette (libQt5Gui.so.5) #6 0x00007fb6613c32d9 _ZN22QGuiApplicationPrivate18notifyThemeChangedEv (libQt5Gui.so.5) #7 0x00007fb6619f558a _ZN19QApplicationPrivate18notifyThemeChangedEv (libQt5Widgets.so.5) #8 0x00007fb6613c2d70 _ZN22QGuiApplicationPrivate19processThemeChangedEPN29QWindowSystemInterfacePrivate16ThemeChangeEventE (libQt5Gui.so.5) #9 0x00007fb6613ca390 _ZN22QGuiApplicationPrivate24processWindowSystemEventEPN29QWindowSystemInterfacePrivate17WindowSystemEventE (libQt5Gui.so.5) #10 0x00007fb6613a4afc _ZN22QWindowSystemInterface22sendWindowSystemEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE (libQt5Gui.so.5) #11 0x00007fb65b94412c n/a (libQt5XcbQpa.so.5) #12 0x00007fb65e82f3ee g_main_context_dispatch (libglib-2.0.so.0) #13 0x00007fb65e831201 n/a (libglib-2.0.so.0) #14 0x00007fb65e831241 g_main_context_iteration (libglib-2.0.so.0) #15 0x00007fb660feaa03 _ZN20QEventDispatcherGlib13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE (libQt5Core.so.5) #16 0x00007fb660f914ec _ZN10QEventLoop4execE6QFlagsINS_17ProcessEventsFlagEE (libQt5Core.so.5) #17 0x00007fb660f99326 _ZN16QCoreApplication4execEv (libQt5Core.so.5) #18 0x0000557cd00a6475 n/a (systemsettings5) #19 0x00007fb660978153 __libc_start_main (libc.so.6) #20 0x0000557cd00a64fe _start (systemsettings5) *** This bug has been marked as a duplicate of bug 412692 ***