Created attachment 187116 [details]
The stack trace.

SUMMARY
After the upgrade from qt6 version 6.10.0  to 6.10.1, and reboot, the plasma has crashed, resulting in infinite crash - relaunch cycle, because the crash was happening consistently. Removing `~/.config` directory allowed to load plasma successfully.
I have found that the fail is caused by the national holidays (tested in US). Removing the next setting from the `plasma-org.kde.plasma.desktop-appletsrc` restores plasma. 
```
[Containments][2][Applets][8][Configuration][Appearance]
enabledCalendarPlugins=holidaysevents
fontWeight=400
```
Enabling holidays in the widget (Settings>Calendar>Holidays) will result in plasma crash; the setting will not be written to the config.

STEPS TO REPRODUCE
1. Right click "Digital clock" widget and select "Settings".
2. In the left panel select "Calendar".
3. Set the check box next to "Holidays"

OBSERVED RESULT
The plasma will crash (segmentation fault).

EXPECTED RESULT
The setting should modify the configuration file; national and regional holidays should be shown up on the calendar.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora release 43 (Forty Three)/plasma-workspace-6.5.3-1.fc43
KDE Plasma Version: 6.5.3
KDE Frameworks Version: 17.08.3
Qt Version: 6.10.1-1

ADDITIONAL INFORMATION
The stack trace is in the attachment file. Here is the informative part:
```JSON
{   "address": 140251518510660
                ,   "build_id": "3abe7149cf37625f289b2094decdc9edb9d57380"
                ,   "build_id_offset": 1091140
                ,   "function_name": "QMetaObject::methodOffset() const"
                ,   "file_name": "/lib64/libQt6Core.so.6"
                }
              , {   "address": 140251518514072
                ,   "build_id": "3abe7149cf37625f289b2094decdc9edb9d57380"
                ,   "build_id_offset": 1094552
                ,   "function_name": "QMetaObject::method(int) const"
                ,   "file_name": "/lib64/libQt6Core.so.6"
                }
              , {   "address": 140251540850393
                ,   "build_id": "808cb4a06d4d1e9980a3e34493ca5d4944c05706"
                ,   "build_id_offset": 2459353
                ,   "function_name": "QQmlPrivate::callQObjectMethod(QV4::ExecutionEngine*, QV4::Lookup*, QObject*, void**, int)"
                ,   "file_name": "/lib64/libQt6Qml.so.6"
                }
              , {   "address": 140250010812296
                ,   "build_id": "939fa404230e02204ad25ab0e6652cb130246df3"
                ,   "build_id_offset": 221064
                ,   "function_name": "QmlCacheGeneratedCode::_qt_qml_org_kde_plasma_workspace_calendar_DayDelegate_qml::aotBuiltFunctions::{lambda(QQmlPrivate::AOTCompiledContext const*, void**)#23}::_FUN(QQmlPrivate::AOTCompiledContext const*, void**) [clone .lto_priv.0]"
                ,   "file_name": "/usr/lib64/qt6/qml/org/kde/plasma/workspace/calendar/libcalendarplugin.so"
                }
              , {   "address": 140251540646334
                ,   "build_id": "808cb4a06d4d1e9980a3e34493ca5d4944c05706"
                ,   "build_id_offset": 2255294
                ,   "function_name": "void QV4::coerceAndCall<(anonymous namespace)::AOTCompiledMetaMethod, QV4::Moth::VME::exec(QV4::MetaTypesStackFrame*, QV4::ExecutionEngine*)::{lambda(void**, int)#1}>(QV4::ExecutionEngine*, (anonymous namespace)::AOTCompiledMetaMethod const*, void**, QMetaType const*, int, QV4::Moth::VME::exec(QV4::MetaTypesStackFrame*, QV4::ExecutionEngine*)::{lambda(void**, int)#1})"
                ,   "file_name": "/lib64/libQt6Qml.so.6"
                }
              , {   "address": 140251540681416
                ,   "build_id": "808cb4a06d4d1e9980a3e34493ca5d4944c05706"
                ,   "build_id_offset": 2290376
                ,   "function_name": "QV4::Moth::VME::exec(QV4::MetaTypesStackFrame*, QV4::ExecutionEngine*)"
                ,   "file_name": "/lib64/libQt6Qml.so.6"
                }
```