[OJS 3.1.2.1] Cannot add any existing user to a new journal

Hello there !!

I’m facing a strange problem when trying to add an existing user to a journal in a multi-journal installation.
As the site manager I go to the new journal -> Users and Roles and do the search … the thing is… I never get any results. Only a never ending spinning circle.

I’ve checked the logs (Apache and MySQL) and the browser console but no error is reported at the moment of executing the search.

Any clue that might point me in the right direction to solve this?

Version OJS 3.1.2.1