Python crash together with threads

My  Software project  is working fine in most of the cases
(www.pythonscad.org)
however I am right now isolating a scenario, which makes it crash
permanently.

It does not happen with Python 3.11.6 (and possibly below), it happens with
3.12 and above
It does not happen when not using Threads.

However due to the architecture of the program I am forced to evaluate some
parts in main thread and some parts in a dedicated Thread. The Thread is
started with QThread(QT 5.0)
whereas I am quite sure that program flows do not overlap.

When I just execute my 1st very simple Python function inside the newly
created thread, like:

 PyObject *a = PyFloat_FromDouble(3.3);

my program crashes with this Stack trace

0  0x00007f6837fe000f in _PyInterpreterState_GET () at
./Include/internal/pycore_pystate.h:179
#1  get_float_state () at Objects/floatobject.c:38
#2  PyFloat_FromDouble (fval=3.2999999999999998) at
Objects/floatobject.c:136
#3  0x00000000015a021f in python_testfunc() ()
#4  0x0000000001433301 in CGALWorker::work() ()
#5  0x0000000000457135 in CGALWorker::qt_static_metacall(QObject*,
QMetaObject::Call, int, void**) ()
#6  0x00007f68364d0f9f in void doActivate<false>(QObject*, int, void**) ()
at /lib64/libQt5Core.so.5
#7  0x00007f68362e66ee in QThread::started(QThread::QPrivateSignal) () at
/lib64/libQt5Core.so.5
#8  0x00007f68362e89c4 in QThreadPrivate::start(void*) () at
/lib64/libQt5Core.so.5
#9  0x00007f6835cae19d in start_thread () at /lib64/libc.so.6
#10 0x00007f6835d2fc60 in clone3 () at /lib64/libc.so.6


I suspect, that this is a Null pointer here
   See also _PyInterpreterState_Get()
   and _PyGILState_GetInterpreterStateUnsafe(). */
static inline PyInterpreterState* _PyInterpreterState_GET(void) {
    PyThreadState *tstate = _PyThreadState_GET();
#ifdef Py_DEBUG
    _Py_EnsureTstateNotNULL(tstate);
#endif
# <<----------- suspect state is nullpointer
    return tstate->interp;
}

any clues , whats going on here, and how I can mitigate that ?