[Closed] Impossible to add translator

Background of the issue:
I am trying to add a translator on a subsite of a multisite WordPress setup using WPML. I have verified that all users have administrator rights.

Symptoms:
When I try to add a translator by writing the name or the admin account name, the pop-up does not propose them. It only worked for one user at the beginning, wpml propose an others ones but not all i neeed and now no user propositions appear.

Questions:
Why does the pop-up not propose users when adding a translator on a subsite?
Could there be an issue specific to this subsite that affects adding translators?