[Chiuso] Can't add user as translator in Translation Dashboard

Background of the issue:
I'm trying to add an existing user as a translator in the WPML Translation Dashboard on my site link nascosto. Clicking 'Add a Translator' opens a popup where I can choose 'Select an existing user and set as Translator'.

Symptoms:
The corresponding user cannot be found, neither by user name, e-Mail, or name.

Questions:
Why can't I find the existing user to set as a translator in WPML?