SUMMARY
Built Plasma 6 with kdesrc-build. Plasmashell immediately crashes. This is what I could get with gdb:

#0  __strcmp_avx2 () at ../sysdeps/x86_64/multiarch/strcmp-avx2.S:287
#1  0x00007ffff4b4430e in QMetaObject::indexOfClassInfo(char const*) const () at /usr/lib/libQt6Core.so.6
#2  0x00007ffff6acc438 in QQmlMetaType::registerType(QQmlPrivate::RegisterType const&) () at /usr/lib/libQt6Qml.so.6
#3  0x00007ffff6a5bc20 in QQmlPrivate::qmlregister(QQmlPrivate::RegistrationType, void*) () at /usr/lib/libQt6Qml.so.6
#4  0x00007fff9405d95b in qmlRegisterType<PipeWireSourceItem>(char const*, int, int, char const*)
    (qmlName=0x7fff940616fc "PipeWireSourceItem", versionMinor=1, versionMajor=0, uri=0x7fff9c5f6e10 "org.kde.taskmanager") at /usr/include/qt6/QtQml/qqml.h:328
#5  TaskManager::TaskManagerPlugin::registerTypes(char const*) (this=<optimized out>, uri=0x7fff9c5f6e10 "org.kde.taskmanager")
    at /home/lucadev/kde/src/plasma-workspace/libtaskmanager/declarative/taskmanagerplugin.cpp:37
#6  0x00007ffff6acdabd in QQmlMetaType::registerPluginTypes(QObject*, QString const&, QString const&, QString const&, QTypeRevision, QList<QQmlError>*) () at /usr/lib/libQt6Qml.so.6
#7  0x00007ffff6af7e03 in  () at /usr/lib/libQt6Qml.so.6
#8  0x00007ffff6aaf168 in QQmlImports::importExtension(QString const&, QTypeRevision, QQmlImportDatabase*, QQmlTypeLoaderQmldirContent const*, QList<QQmlError>*) () at /usr/lib/libQt6Qml.so.6
#9  0x00007ffff6ab9dbd in QQmlImports::addLibraryImport(QQmlImportDatabase*, QString const&, QString const&, QTypeRevision, QString const&, QString const&, QFlags<QQmlImports::ImportFlag>, unsigned short, QList<QQmlError>*) () at /usr/lib/libQt6Qml.so.6
#10 0x00007ffff6b4c814 in  () at /usr/lib/libQt6Qml.so.6
#11 0x00007ffff6b49d66 in QQmlTypeLoader::Blob::addLibraryImport(std::shared_ptr<QQmlTypeLoader::Blob::PendingImport> const&, QList<QQmlError>*) () at /usr/lib/libQt6Qml.so.6
#12 0x00007ffff6b4b379 in QQmlTypeLoader::Blob::addImport(QV4::CompiledData::Import const*, QFlags<QQmlImports::ImportFlag>, QList<QQmlError>*) () at /usr/lib/libQt6Qml.so.6
#13 0x00007ffff6b42071 in  () at /usr/lib/libQt6Qml.so.6
#14 0x00007ffff6b4554a in QQmlTypeLoader::setData(QQmlRefPointer<QQmlDataBlob> const&, QQmlDataBlob::SourceCodeData const&) () at /usr/lib/libQt6Qml.so.6
#15 0x00007ffff6b45aa1 in QQmlTypeLoader::setData(QQmlRefPointer<QQmlDataBlob> const&, QString const&) () at /usr/lib/libQt6Qml.so.6
#16 0x00007ffff6b486ea in QQmlTypeLoader::loadThread(QQmlRefPointer<QQmlDataBlob> const&) () at /usr/lib/libQt6Qml.so.6
#17 0x00007ffff6b4d559 in  () at /usr/lib/libQt6Qml.so.6
#18 0x00007ffff6a54dcb in  () at /usr/lib/libQt6Qml.so.6
#19 0x00007ffff5f7134b in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/lib/libQt6Widgets.so.6
#20 0x00007ffff4b3ba98 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/libQt6Core.so.6
#21 0x00007ffff4b3be1b in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /usr/lib/libQt6Core.so.6
#22 0x00007ffff4d3dd34 in  () at /usr/lib/libQt6Core.so.6
#23 0x00007ffff3c40f19 in  () at /usr/lib/libglib-2.0.so.0
#24 0x00007ffff3c9f2b7 in  () at /usr/lib/libglib-2.0.so.0
#25 0x00007ffff3c3f112 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#26 0x00007ffff4d3bd24 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt6Core.so.6
#27 0x00007ffff4b45953 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt6Core.so.6
#28 0x00007ffff4c2f43f in QThread::run() () at /usr/lib/libQt6Core.so.6
#29 0x00007ffff4c74932 in  () at /usr/lib/libQt6Core.so.6
#30 0x00007ffff44aa9eb in start_thread (arg=<optimized out>) at pthread_create.c:444
#31 0x00007ffff452e7cc in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78

These are the logs printer by plasmashell:

kf.svg: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
kf.svg: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
kf.plasma.core: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
kf.plasma.core: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
KPackageStructure of KPluginMetaData(pluginId:"org.kde.kdeconnect", fileName: "/usr/share/plasma/plasmoids/org.kde.kdeconnect/metadata.json") does not match requested format "Plasma/Applet"
KPackageStructure of KPluginMetaData(pluginId:"org.kde.plasma.simplemenu", fileName: "/usr/share/plasma/plasmoids/org.kde.plasma.simplemenu/metadata.json") does not match requested format "Plasma/Applet"
KPackageStructure of KPluginMetaData(pluginId:"org.kde.plasma.printmanager", fileName: "/usr/share/plasma/plasmoids/org.kde.plasma.printmanager/metadata.json") does not match requested format "Plasma/Applet"
KPackageStructure of KPluginMetaData(pluginId:"org.manjaro.desktop.breathPanel", fileName: "/usr/share/plasma/layout-templates/org.manjaro.desktop.breathPanel/metadata.json") does not match requested format "Plasma/LayoutTemplate"
qt.qml.typeregistration: Invalid QML element name "OsdAction"; value type names should begin with a lowercase letter
kf.svg: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
kf.plasma.quick: Applet preload policy set to 1
file:///home/lucadev/kde/usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/main.qml:93: TypeError: Cannot call method 'elementSize' of null
file:///home/lucadev/kde/usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/main.qml:192:25: QML FolderViewDropArea (parent or ancestor of QQuickLayoutAttached): Binding loop detected for property "minimumWidth"
qt.accessibility.atspi: Error in contacting registry: "org.freedesktop.DBus.Error.Disconnected" "Not connected to D-Bus server"
KCrash: Attempting to start /home/lucadev/kde/usr/bin/plasmashell
KCrash: Application 'plasmashell' crashing...
The Wayland connection experienced a fatal error: Bad file descriptor
kf.svg: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.                              1 ✘ 
kf.svg: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
kf.plasma.core: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
kf.plasma.core: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
KPackageStructure of KPluginMetaData(pluginId:"org.kde.kdeconnect", fileName: "/usr/share/plasma/plasmoids/org.kde.kdeconnect/metadata.json") does not match requested format "Plasma/Applet"
KPackageStructure of KPluginMetaData(pluginId:"org.kde.plasma.simplemenu", fileName: "/usr/share/plasma/plasmoids/org.kde.plasma.simplemenu/metadata.json") does not match requested format "Plasma/Applet"
KPackageStructure of KPluginMetaData(pluginId:"org.kde.plasma.printmanager", fileName: "/usr/share/plasma/plasmoids/org.kde.plasma.printmanager/metadata.json") does not match requested format "Plasma/Applet"
KPackageStructure of KPluginMetaData(pluginId:"org.manjaro.desktop.breathPanel", fileName: "/usr/share/plasma/layout-templates/org.manjaro.desktop.breathPanel/metadata.json") does not match requested format "Plasma/LayoutTemplate"
qt.qml.typeregistration: Invalid QML element name "OsdAction"; value type names should begin with a lowercase letter
kf.svg: The theme "breath-dark" uses the legacy metadata.desktop. Consider contacting the author and asking them update it to use the newer JSON format.
kf.plasma.quick: Applet preload policy set to 1
file:///home/lucadev/kde/usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/main.qml:93: TypeError: Cannot call method 'elementSize' of null
file:///home/lucadev/kde/usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/main.qml:192:25: QML FolderViewDropArea (parent or ancestor of QQuickLayoutAttached): Binding loop detected for property "minimumWidth"
qt.accessibility.atspi: Error in contacting registry: "org.freedesktop.DBus.Error.Disconnected" "Not connected to D-Bus server"

At the beginning I ignored kpipewire, cause my system is missing pipewire-audio. Then I also tried to rebuild everything with kpipewire. Nothing changed.

Is it possible to build without pipewire? As soon as I install it, I have problems with audio.

STEPS TO REPRODUCE
1. Build Plasma 6 workspace with kdesrc-build, without any additional configuration.
2. Run the configuration script to be able to login from sddm.
3. Login.

OBSERVED RESULT
The getting started window appears, but plasmashell keeps crashing.

EXPECTED RESULT
No crash should occur.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Plasma 6 built from kdesrc-build
Qt Version: 6.5.3

ADDITIONAL INFORMATION
System is fully updated Manjaro.