Problem of database connection to localhost with FlameRobin/Firebird

Question

I'm coming here because I wanted to test some things with Firebird.
Indeed, I need to create a database with LibreOffice Base, which now comes with Firebird-3:
I would like to not have an embedded database in LibreOffice Base, but better connect LO Base to a Firebird file. So that, LibreOffice is just an interface and database management tool.
And as a result, I would get a fdb file that I could access from other applications, such as isql, or FlameRobin.

Here comes my problem: when trying to connect a database through localhost (with both isql and FlameRobin), with the sysbda username and password, I get this error message:

isql-fb 
Use CONNECT or CREATE DATABASE to specify a database
SQL> connect localhost:test.fdb user sysbda password mdptestmdp;
Statement failed, SQLSTATE = 28000
Your user name and password are not defined. Ask your database administrator to set up a Firebird login.

or in FlameRobin:

*** IBPP::SQLException ***
Context: Database::Connect
Message: isc_attach_database failed

SQL Message : -902
Unsuccessful execution caused by a system error that precludes successful execution of subsequent statements

Engine Code    : 335544472
Engine Message :
Your user name and password are not defined. Ask your database administrator to set up a Firebird login.

For information,

• when I access the databases not through the localhost server, but with the usual path, it works.
• I changed, checked and re-checked my sysbda username and password.
• I tested with one new user created with right-click on 'localhost' in FlameRobin, and it doesn't change anything.
• I tested with one database fdb file created from the LO Base database creation wizard (connect to a Firebird file), and with a database created within the SQL prompt of the isql-fb tool
• I run on Lubuntu 18.04, with LibreOffice 6.3, and installed Firebird3.0 and FlameRobin from default Ubuntu packages (sudo apt install flamerobin and sudo apt install firebird3*)

I really don't know how to do... I've try to find out, but did not find any solution for that...
I am still poorly skilled in such computer issue... and have never worked before with server aspects... maybe should I also configure my "localhost"...? (I am connected to internet only through metered 4G).

Please ask me for any further detail that could help to fix that.

EDIT
Following your comments, Mark Rorreveel, here are some new informations:

• connect localhost:test.fdb user sysbda password 'mdptestmdp'; doesn't change anything...
• I checked my username and password in the SYSBDA.password file and they are the indicated ones.
• #AuthServer = Srp. Should I remove the #?
• #AuthClient = Srp, Srp256, Legacy_Auth #Non Windows clients. As well # to be removed?
• #UserManager = Srp. As well again to be removed the #?