It is (was) impossible to log into the repository or any other module, including Admin Console ("We don´t have access to any LF app i.e. client, web client, admin console... ") i.e. we could not even see the repository and therefore, we could not change the DBMS... so, the real question here is if there is another method to actually see the DB user that LF is using to connect to the repo DB.
At the end, problem was that the user that was being used to connect to the DB had a password expiration policy enabled (we don´t know how that happened or who...¿?) and it had expired.
We were able to see who the LF DB user was, in the event viewer in the LF ContentRepository section under Applications and Services Reg. BUT I´d like to know if there is a place where this information is kept, be it a .config or a .xml file somewhere in the LF install.