after trying to create/edit users and roles I've accidentally locked myself out of the users management page, which now returns "You don't have permissions to view the required screen. Please contact the system administrator.".
I would recommend you to open a ticket with Support (https://www.outsystems.com/SupportPortal) so that you can get the help you need!
I talked with Joana from the OutSystems team, I hadn't realized I removed myself from the UserManager role. All I had to do was create a new User_role and everything is working fine.