Toms, thank you! There is an interesting thing happening:
I added a display object on the main form to display the value of '#MAIN_FORM_RECORD_ID#'. When I enter the form in edit mode, the display object is empty (called "test" in the following screenshot), and thus my subform select object is empty too:
Capture d’écran 2018-05-28 à 17.06.19.png
But then when I click on Arrange objects in the options, that I move the display object containing the value of '#MAIN_FORM_RECORD_ID#', and I save, the display object works and the select object in the subform works as well:
Capture d’écran 2018-05-28 à 17.05.54.png
When I leave the form and come back, again nothing shows in the display and select objects.
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