SUMMARY
This was also reported here: https://bugzilla.suse.com/show_bug.cgi?id=1246411

This morning I've performed a "zypper up" and it picked up some updates from OBS KDE/Plasma repos (looks like KDE Apps update to 6.16 and some libQt6Quick3D updates and sddm-qt6-branding rpm). Now, when sddm starts you see the login screen, mouse moves and that's it. It is impossible to select a user to login nor enter a password. 

After a bit of poking around, I have noticed that sddm-greeter-qt6 is crashing with the following stack trace (full gdb info attached). 

GNU gdb (GDB; SUSE Linux Enterprise 15) 14.2
...
Reading symbols from /usr/bin/sddm-greeter-qt6...
Reading symbols from /home/data/usr/lib/debug/usr/bin/sddm-greeter-qt6-0.21.0-lp156.156.1.x86_64.debug...
...
Core was generated by `/usr/bin/sddm-greeter-qt6 --socket /tmp/sddm-:0-LIPVgP --theme /usr/share/sddm/'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007fae6a51b118 in QQmlPrivate::AOTCompiledContext::getEnumLookup (this=this@entry=0x55686795fd90, index=index@entry=23, target=target@entry=0x7fffdf48998c)
    at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/qml/qml/qqml.cpp:2814
2814	    if (lookup->call != QV4::Lookup::Call::GetterEnumValue)
[Current thread is 1 (Thread 0x7fae6701ea80 (LWP 2921))]

(gdb) where
#0  0x00007fae6a51b118 in QQmlPrivate::AOTCompiledContext::getEnumLookup (this=this@entry=0x55686795fd90, index=index@entry=23, target=target@entry=0x7fffdf48998c)
    at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/qml/qml/qqml.cpp:2814
#1  0x00007fae628c4319 in QmlCacheGeneratedCode::_qt_qml_org_kde_kirigami_primitives_Separator_qml::aotBuiltFunctions::{lambda(QQmlPrivate::AOTCompiledContext const*, void**)#3}::operator()(QQmlPrivate::AOTCompiledContext const*, void**) const (__closure=<optimized out>, aotContext=0x55686795fd90, argv=0x556867d0f280)
    at /usr/src/debug/kf6-kirigami-6.16.0-kf.49.1.x86_64/build/src/primitives/.rcc/qmlcache/KirigamiPrimitives_Separator_qml.cpp:685
#2  0x00007fae6ad25a8a in QSGSoftwareRenderableNode::update (this=0x556867d0f380)
    at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/adaptations/software/qsgsoftwarerenderablenode.cpp:151
#3  0x00007fae6ad17e15 in QSGAbstractSoftwareRenderer::nodeChanged (this=0x5568679f2310, node=0x556867d0f280, state=...)
    at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/adaptations/software/qsgabstractsoftwarerenderer.cpp:66
#4  0x00007fae6ad5f1c8 in QSGRootNode::notifyNodeChange (this=this@entry=0x55686786c3f0, node=node@entry=0x556867d0f280, state=state@entry=...)
    at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/coreapi/qsgnode.cpp:1242
#5  0x00007fae6ad5f250 in QSGNode::markDirty (this=0x556867d0f280, bits=...) at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/coreapi/qsgnode.cpp:636
#6  0x00007fae628be687 in SoftwareRectangleNode::preprocess (this=0x5568679604c0) at /usr/src/debug/kf6-kirigami-6.16.0-kf.49.1.x86_64/src/primitives/scenegraph/softwarerectanglenode.cpp:134
#7  0x00007fae6ad62e53 in QSGRenderer::preprocess (this=0x5568679f2310) at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/coreapi/qsgrenderer.cpp:239
#8  0x00007fae6ad63335 in QSGRenderer::renderScene (this=0x5568679f2310) at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/coreapi/qsgrenderer.cpp:142
#9  QSGRenderer::renderScene (this=0x5568679f2310) at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/coreapi/qsgrenderer.cpp:120
#10 0x00007fae6ad02adb in QQuickWindowPrivate::renderSceneGraph (this=this@entry=0x556866cbc2c0) at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/items/qquickwindow.cpp:690
#11 0x00007fae6ad2e10c in QSGSoftwareRenderLoop::renderWindow (this=0x556866c9a810, window=<optimized out>, isNewExpose=false)
    at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/scenegraph/adaptations/software/qsgsoftwarerenderloop.cpp:145
#12 0x00007fae6ad06c85 in QQuickWindow::event (this=<optimized out>, event=<optimized out>) at /usr/src/debug/qt6-declarative-6.9.1-lp156.110.7.x86_64/src/quick/items/qquickwindow.cpp:1590
#13 0x00007fae68fa34e8 in QCoreApplication::notifyInternal2 (receiver=0x556866c9cd70, event=0x7fffdf48a0d0)
    at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/kernel/qcoreapplication.cpp:1106
#14 0x00007fae68fa352d in QCoreApplication::sendEvent (receiver=<optimized out>, event=<optimized out>)
    at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/kernel/qcoreapplication.cpp:1546
#15 0x00007fae69839b75 in QPlatformWindow::deliverUpdateRequest (this=<optimized out>) at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/gui/kernel/qplatformwindow.cpp:831
#16 0x00007fae698394ae in QPlatformWindow::windowEvent (this=0x556867bfd260, event=<optimized out>) at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/gui/kernel/qplatformwindow.cpp:472
#17 0x00007fae697f8873 in QGuiApplication::notify (this=0x7fffdf48a530, object=0x556866c9cd70, event=0x7fffdf48a1f0)
    at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/gui/kernel/qguiapplication.cpp:2049
#18 0x00007fae68fa34e8 in QCoreApplication::notifyInternal2 (receiver=0x556866c9cd70, event=0x7fffdf48a1f0)
    at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/kernel/qcoreapplication.cpp:1106
#19 0x00007fae69106ada in QTimerInfoList::activateTimers (this=0x556866c8e570) at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/kernel/qtimerinfo_unix.cpp:426
#20 0x00007fae691e6cac in timerSourceDispatch (source=<optimized out>) at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/kernel/qeventdispatcher_glib.cpp:152
#21 0x00007fae68114f9c in g_main_dispatch (context=0x7fae5c000ef0) at ../glib/gmain.c:3476
#22 g_main_context_dispatch_unlocked (context=context@entry=0x7fae5c000ef0) at ../glib/gmain.c:4284
#23 0x00007fae68116bc0 in g_main_context_iterate_unlocked (context=context@entry=0x7fae5c000ef0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/gmain.c:4349
#24 0x00007fae681171bc in g_main_context_iteration (context=0x7fae5c000ef0, may_block=1) at ../glib/gmain.c:4414
#25 0x00007fae691e4490 in QEventDispatcherGlib::processEvents (this=0x556866c7eb80, flags=...) at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/kernel/qeventdispatcher_glib.cpp:399
#26 0x00007fae68fad6bb in QEventLoop::exec (this=0x7fffdf48a460, flags=...) at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/global/qflags.h:77
#27 0x00007fae68fa6ee8 in QCoreApplication::exec () at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/corelib/global/qflags.h:77
#28 0x00007fae697f8760 in QGuiApplication::exec () at /usr/src/debug/qt6-base-6.9.1-lp156.155.1.x86_64/src/gui/kernel/qguiapplication.cpp:1986
#29 0x000055685cb3f2ba in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/sddm-qt6-0.21.0-lp156.156.1.x86_64/src/greeter/GreeterApp.cpp:394

(gdb) p lookup
$1 = (QV4::Lookup *) 0x3d9

(gdb) p *this
$2 = {
  qmlContext = 0x7fae628dfec8 <vtable for std::_Sp_counted_deleter<QSGTexture*, TextureCache::loadTexture(QQuickWindow*, QImage const&, QFlags<QQuickWindow::CreateTextureOption>)::{lambda(QSGTexture*)#1}, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>+16>, qmlScopeObject = 0x200000001, engine = 0x1eb00000003, {compilationUnit = 0x556866c9cd70, extraData = 93906889461104}}

I've attempted to purge /var/lib/sddm/.cache but that made no difference.

Reproducible: Always

Actual Results:  
sddm-greeter-qt6 crashes, impossible to login

Expected Results:  
sddm-greeter-qt6 does not crash, users can login

SOFTWARE/OS VERSIONS
Operating System: openSUSE Leap 15.6
KDE Plasma Version: 6.4.2
KDE Frameworks Version: 6.16.0
Qt Version: 6.9.1
Kernel Version: 6.4.0-150600.23.53-default (64-bit)
Graphics Platform: X11
Graphics Processor: NVIDIA GeForce RTX 4060 Ti

ADDITIONAL INFORMATION
$ zypper lr --uri | grep KDE
 4 | KDE:Applications_openSUSE_Leap_15.6           | KDE:Applications_openSUSE_Leap_15.6                          | Yes     | (r ) Yes  | Yes     | https://cdn.opensuse.org/repositories/KDE:/Applications/KDE_Frameworks5_openSUSE_Leap_15.6/
 5 | KDE:Extra_openSUSE_Leap_15.6                  | KDE:Extra_openSUSE_Leap_15.6                                 | Yes     | (r ) Yes  | Yes     | https://cdn.opensuse.org/repositories/KDE:/Extra/KDE_Applications_openSUSE_Leap_15.6/
 6 | KDE:Frameworks5_openSUSE_Leap_15.6            | KDE:Frameworks5_openSUSE_Leap_15.6                           | Yes     | (r ) Yes  | Yes     | https://cdn.opensuse.org/repositories/KDE:/Frameworks5/openSUSE_Leap_15.6/
 7 | KDE:Frameworks_openSUSE_Leap_15.6             | KDE:Frameworks_openSUSE_Leap_15.6                            | Yes     | (r ) Yes  | Yes     | https://cdn.opensuse.org/repositories/KDE:/Frameworks/openSUSE_Leap_15.6/
 8 | KDE:Qt5_openSUSE_Leap_15.6                    | KDE:Qt5_openSUSE_Leap_15.6                                   | Yes     | (r ) Yes  | Yes     | https://cdn.opensuse.org/repositories/KDE:/Qt5/openSUSE_Leap_15.6
 9 | KDE:Qt6_openSUSE_Leap_15.6                    | KDE:Qt6_openSUSE_Leap_15.6                                   | Yes     | (r ) Yes  | Yes     | https://cdn.opensuse.org/repositories/KDE:/Qt6/openSUSE_Leap_15.6

Full list of installed updates this morning is attached. Looks like there were some libQt6Quick updates.

$ grep -i qt6 latest_obs_updates.txt
sddm-qt6-debuginfo-0.21.0-lp156.156.1.x86_64  Sat 12 Jul 2025 08:34:38 BST
sddm-greeter-qt6-debuginfo-0.21.0-lp156.156.1.x86_64 Sat 12 Jul 2025 08:34:38 BST
sddm-qt6-debugsource-0.21.0-lp156.156.1.x86_64 Sat 12 Jul 2025 08:34:37 BST
qt6-quick3d-imports-6.9.1-lp156.73.5.x86_64   Sat 12 Jul 2025 08:18:45 BST
libQt6Quick3DHelpersImpl6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:45 BST
libQt6Quick3DHelpers6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:45 BST
libQt6Quick3DAssetUtils6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:45 BST
libQt6Quick3DXr6-6.9.1-lp156.73.5.x86_64      Sat 12 Jul 2025 08:18:44 BST
libQt6Quick3DUtils6-6.9.1-lp156.73.5.x86_64   Sat 12 Jul 2025 08:18:44 BST
libQt6Quick3DRuntimeRender6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:44 BST
libQt6Quick3DParticles6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:44 BST
libQt6Quick3D6-6.9.1-lp156.73.5.x86_64        Sat 12 Jul 2025 08:18:44 BST
libQt6Quick3DParticleEffects6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:43 BST
libQt6Quick3DEffects6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:43 BST
libQt6Quick3DAssetImport6-6.9.1-lp156.73.5.x86_64 Sat 12 Jul 2025 08:18:43 BST
libKF6NetworkManagerQt6-debuginfo-6.16.0-kf.35.1.x86_64 Sat 12 Jul 2025 08:18:26 BST
libKF6BluezQt6-debuginfo-6.16.0-kf.38.1.x86_64 Sat 12 Jul 2025 08:18:12 BST
libKF6NetworkManagerQt6-6.16.0-kf.35.1.x86_64 Sat 12 Jul 2025 08:18:03 BST
libKF6ModemManagerQt6-debuginfo-6.16.0-kf.35.1.x86_64 Sat 12 Jul 2025 08:18:03 BST
libKF6ModemManagerQt6-6.16.0-kf.35.1.x86_64   Sat 12 Jul 2025 08:18:03 BST
sddm-qt6-branding-openSUSE-6.4.2-kf.86.3.x86_64 Sat 12 Jul 2025 08:17:54 BST
libKF6BluezQt6-6.16.0-kf.38.1.x86_64          Sat 12 Jul 2025 08:17:54 BST

Something else I've noticed is that as a result of installing sddm-qt6-branding-openSUSE-6.4.2 the contents of 10-plasma.conf and 11-kwin_wayland.conf in /usr/lib/sddm/sddm.conf.d are very similar.

$ cat 10-plasma.conf
[General]
GreeterEnvironment=QT_WAYLAND_SHELL_INTEGRATION=layer-shell

[Theme]
Current=breeze-openSUSE
CursorTheme=breeze_cursors

[Wayland]
CompositorCommand=kwin_wayland --no-global-shortcuts --no-lockscreen --locale1

$ cat 11-kwin_wayland.conf
[General]
GreeterEnvironment=QT_WAYLAND_SHELL_INTEGRATION=layer-shell

[Wayland]
# --locale1 was introduced in kwin 5.27
CompositorCommand=kwin_wayland --no-global-shortcuts --no-lockscreen --locale1