I would suggest that this is probably an issue with the downgrade process. Does it happen with a fresh install of 5.22.x or an upgrade install with no downgrade?

Only happened after the downgrade (not before and not on another system).

Probably as a side-effect krunner does not open and the screen for logout/shutdown does not open.

gdb backtrace:


#0  QObject::setProperty (this=this@entry=0x0, name=name@entry=0x55555555b788 "stage", value=...) at kernel/qobject.cpp:4030
        d = <optimized out>
        meta = <optimized out>
        id = <optimized out>
        p = {mobj = 0x6e0000005b, handle = 4294957464, idx = 32767, menum = {mobj = 0x555555630b08, handle = 1433196272}}
#1  0x000055555555a605 in SplashWindow::setStage (this=this@entry=0x5555556bca80, stage=0) at ./ksplash/ksplashqml/splashwindow.cpp:88
No locals.
#2  0x000055555555921d in SplashApp::adoptScreen (this=this@entry=0x7fffffffdba0, screen=0x555555630ab0) at ./ksplash/ksplashqml/splashapp.cpp:142
        w = 0x5555556bca80
#3  0x0000555555559a0f in SplashApp::SplashApp (this=0x7fffffffdba0, argc=<optimized out>, argv=<optimized out>) at ./ksplash/ksplashqml/splashapp.cpp:77
        screen = <optimized out>
        _container_ = {c = {<QListSpecialMethods<QScreen*>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, 
                  end = 0, array = {0x0}}, d = <optimized out>}, d = <optimized out>}}, i = {i = <optimized out>}, e = {i = <optimized out>}, control = <optimized out>}
        parser = {d = 0x55555573e860}
        dbus = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff7953880, data = 0x7ffff7953740, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}, d = 0x7fffc0003a00}
        cursor = <incomplete type>
        parser = {d = <optimized out>}
        dbus = {static staticMetaObject = {d = {superdata = {direct = 0x0}, stringdata = 0x7ffff7953880, data = 0x7ffff7953740, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}, d = <optimized out>}
        cursor = <incomplete type>
        _container_ = {c = {<QListSpecialMethods<QScreen*>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, 
                  end = 0, array = {0x0}}, d = <optimized out>}, d = <optimized out>}}, i = {i = <optimized out>}, e = {i = <optimized out>}, control = <optimized out>}
        screen = <optimized out>
#4  0x00005555555586f2 in main (argc=<optimized out>, argv=0x7fffffffdcf8) at ./ksplash/ksplashqml/main.cpp:69
        test = <optimized out>
        app = {<QGuiApplication> = {<No data fields>}, static staticMetaObject = {d = {superdata = {direct = 0x7ffff786e0a0 <QGuiApplication::staticMetaObject>}, stringdata = 0x55555555b0c0 <qt_meta_stringdata_SplashApp>, 
              data = 0x55555555b020 <qt_meta_data_SplashApp>, static_metacall = 0x555555558840 <SplashApp::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, m_stage = 0, 
          m_windows = {<QListSpecialMethods<SplashWindow*>> = {<No data fields>}, {p = {static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, 
                  end = 0, array = {0x0}}, d = 0x7ffff6ffc470 <QListData::shared_null>}, d = 0x7ffff6ffc470 <QListData::shared_null>}}, m_testing = false, m_window = false, m_stages = {<QList<QString>> = {<QListSpecialMethods<QString>> = {<No data fields>}, {p = {
                  static shared_null = {ref = {atomic = {_q_value = {<std::__atomic_base<int>> = {static _S_alignment = 4, _M_i = -1}, static is_always_lock_free = true}}}, alloc = 0, begin = 0, end = 0, array = {0x0}}, d = 0x7ffff6ffc470 <QListData::shared_null>}, 
                d = 0x7ffff6ffc470 <QListData::shared_null>}}, <No data fields>}, m_timer = {id = 0}, m_theme = {static null = {<No data fields>}, d = 0x7ffff6ffb960 <QArrayData::shared_null>}}

All of these issues are characteristic signs of a failed downgrade, or bugs in the downgrade process itself on Debian, maybe. I would recommend following up with Norbert or other Debian packagers.

After looking at the home directory files opened by ksplashqml (via strace) and removing them, the issue disappeared.

The removed files were:
.cache/icon-cache.kcache
.cache/mesa_shader_cache/
.config/drkonqirc
.config/fontconfig/fonts.conf
.config/kdeglobals
.config/kwinrc
.Xauthority

Probably the cache files. Downgrading should definitely remove those.