Thank you Victor for your response.
I follow your advice but the output was an error in a different point of the code and I couldn't see any information about memory allocations.

But, I compiled Python 3.7 from source and modified the "Python/errors.c" code where the problem is detected in the following way:

void
PyErr_SetObject(PyObject *exception, PyObject *value)
{
    PyThreadState *tstate = PyThreadState_GET();
    PyObject *exc_value;
    PyObject *tb = NULL;
    _PyErr_StackItem *exc_info = NULL;


    if (exception != NULL &&
        !PyExceptionClass_Check(exception)) {
        PyErr_Format(PyExc_SystemError,
                     "exception %R not a BaseException subclass",
                     exception);
        return;
    }

    Py_XINCREF(value);
    exc_info = _PyErr_GetTopmostException(tstate);
    exc_value = exc_info->exc_value;
    if (exc_value != NULL && exc_value != Py_None) {
        /* Implicit exception chaining */
        printf("exc_value=%p\n", exc_value);
        printf("exc_info=%p\n", exc_info);
        printf("tstate=%p\n", tstate);
        printf("traceback=%p\n", exc_info->exc_traceback);
        printf("exc_value.ob_type=%p\n", exc_value->ob_type);
        Py_INCREF(exc_value);
       
In this way I had the pointer printed just before the error happening:

exc_value=0x73726573756e69
exc_info=0x7f83bfafacb8
tstate=0x5605dcd41330
traceback=0x6b6f0100
Segmentation fault (core dumped)

And by using gdb I printed the memory contents:

(gdb) x/32c 0x7f83bfafacb8
0x7f83bfafacb8:	111 'o'	115 's'	99 'c'	99 'c'	46 '.'	97 'a'	100 'd'	109 'm'
0x7f83bfafacc0:	105 'i'	110 'n'	117 'u'	115 's'	101 'e'	114 'r'	115 's'	0 '\000'
0x7f83bfafacc8:	0 '\000'	1 '\001'	111 'o'	107 'k'	0 '\000'	0 '\000'	0 '\000'	0 '\000'
0x7f83bfafacd0:	0 '\000'	0 '\000'	0 '\000'	-16 '\360'	63 '?'	0 '\000'	0 '\000'	0 '\000'

If you check the original post you will see:

#15 0x000000000053f732 in PyEval_EvalFrameEx (throwflag=0, 
    f=Frame 0x17882e8, for file /usr/local/lib/python3.7/dist-packages/bson/__init__.py, line 1013, in decode_all (data=b'V\x00\x00\x00\x03cursor\x00=\x00\x00\x00\x04firstBatch\x00\x05\x00\x00\x00\x00\x12id\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02ns\x00\x13\x00\x00\x00gecoscc.adminusers\x00\x00\x01ok\x00\x00\x00\x00\x00\x00\x00\xf0?\x00', codec_options=<CodecOptions at remote 0x7f1ab4ac2208>, view=<memoryview at remote 0x7f1ab5375708>, data_len=86, docs=[], position=0, end=85)) at ../Python/ceval.c:547

So, somehow the "oscc.adminusers\x00\x00\x01ok\x00" part of the message is written over the exc_info memory.

That makes me think that this is a problem in pymongo module. I will report this bug to them.

Thank you very much!