Unsolved QQmlPropertyMap update from c++ not reflected in gui

[UKLU]

Dear Specialists,

I have:

defined static var:
QQmlPropertyMap MasterParameter::theMasterParameterMap;

set up and registered:
theMasterParameterMap["AutoLogoffText"] = "";
theEngine->rootContext()->setContextProperty(APPLICATION_NAME,&theMasterParameterMap);

used:
Label
{
text: KompliKat.AutoLogoffText
color: KompliKat.FontHighLightColor
anchors.centerIn: parent
}

And:
This works perfectly!

But:
But when I update the value from a QTimer callback in c++:

MasterParameter::theMasterParameterMap["AutoLogoffText"] = <<< QString from SQL query >>>;

the qml front end does not display the new value.

The documentation states, that:
The binding is dynamic - whenever a key's value is updated, anything bound to that key will be updated as well.

Am I missing something here? How can I get this to work?
I cannot for the life of me figure out what the Problem is here :-(
Any hints are greatly appreciated! Thank you for your time!

Marc