Given you haven’t provided the information I asked for in the comments, I have to guess you are using Firebird 3 with Jaybird 2.2.x based on the described behavior.
The error connection rejected by remote interface can also occur under other conditions than described below.
Possible cause: wire protocol encryption required
Firebird 3 introduces a number of new security features that are enabled by default, but are not supported by Jaybird 2.2. To allow Jaybird 2.2 to connect to Firebird 3 you need to relax some of those settings.
To allow Jaybird 2.2.x to connect, you need to change the following settings in firebird.conf (and restart Firebird after changing the setting):
Relax the WireCrypt setting from its default of Required to Enabled:
WireCrypt = Enabled
Enable support for the legacy authentication protocol:
AuthServer = Srp, Legacy_Auth
You then need to make sure the user you want to use for connecting to Firebird is created with the legacy usermanager by enabling support for the legacy usermanager:
UserManager = Srp, Legacy_UserManager
Restart Firebird to apply these settings, and then – in Flamerobin – with a SYSDBA account (or user with role RDB$ADMIN), create the required user:
CREATE USER youruser PASSWORD 'yourpasw' USING PLUGIN Legacy_UserManager
Alternatively you could upgrade to Jaybird 3.0.4 or higher, which supports the Srp authentication protocol and wire protocol encryption.
These settings are described in more detail on our wiki in article Jaybird and Firebird 3. This information was absent from our FAQ, I have now added it under connection rejected by remote interface (335544421).
Possible cause: no user name or password
With Jaybird 3 or higher and Firebird 3 or higher, this error can be the result of not providing a username or password. The absence of a user name or password causes Jaybird to not try any authentication plugin. This results in Firebird rejecting the connection attempt as at least one authentication attempt should be done.