Kup-Backup crashes when using Background Opacity via the QT-Curve Widget Style. It crashes even when using 95% opacity.
Confirmed that I see the crash in the opening the kup settings module (run kcmshell5 kcm_kup). Did not try any other parts of kup. Tried looking at valgrind output and also a core dump with gdb. I have no idea what is going on there. If somebody feels like building all of kde and qt with debug symbols and get a useful backtrace, that could be a good start. I don't have time for that myself. Thanks for the report!