Hi Bruno,
Ah, yes, good catch. This happens because the initial value for app is empty, and that value is only set when you select something in the dropdown, but since it contains just one item, you can't change the section, and thus are stuck. I fixed this bug, and the fix will be in the 4.2 which should be released in the next few days.
https://github.com/orbeon/orbeon-forms/issues/1006(For now, as a workaround, make sure users has access to at least 2 apps, and if you wanted the first, in the dialog, choose the second, and then go back to the first. And sorry for the trouble!)
Alex
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