Hi again @martin.stamm,
I decided to remove the “Out Mapping” Variable in my Multi-Instance Call Activity and I ran again my process.
The result was that my merged json variable (“aggregatedList”) is fetched in Cockpit (as I can see all the submitted values from the Subprocess) but it seems that some problem still exists concerning the display of the same variable (“aggregatedList”) in the next task form of Taskllist as the form can’t be loaded there 
Could you please check the following script (from my embedded html file) which concerns the above task form (in which I want to display the “aggregatedList” variable) ?
<script cam-script type="text/form-script">
var variableManager = camForm.variableManager;
camForm.on('form-loaded', function() { // No any variable has been loaded from the server.
variableManager.fetchVariable('aggregatedList'); // We declare the merged json variable 'aggregatedList' so as to fetch its values.
});
camForm.on('variables-fetched', function() { // The merged json variable 'aggregatedList' has been fetched from the server.
aggregatedList[$scope.task.assignee] = selectedProduct;
var variableValue = variableManager.variableValue('aggregatedList', aggregatedList);
});
</script>
Thank you a lot 
Steve