Subject 'connection rejected by remote interface' connection to Firebird 3.0 from Flamerobin and Delphi IBX

We are trying to use Firebird 3.0 for our Delphi programs that use IBX components (Delphi 2009, Delphi 2010 and Delphi XE). Local connection to the Firebird 3.0 database is succesfull both from Flamerobin and from IBX. The problem is connection to the remote server: both - Flamerobin and IBX - give the following error message:

Unhandled Error in FlameRobin
*** IBPP::SQLException ***
Context: Database::Connect
Message: isc_attach_database failed

SQL Message : -923
Connection not established

Engine Code    : 335544421
Engine Message :
connection rejected by remote interface


What should we do? We use Firebird 3.0 client (fbclient.dll and gds32.dll) so there is no incompatibility and this should not be an issue. We tried to change Firebird.config parameters:

AuthServer = Legacy_Auth, Srp, Win_Sspi

AuthClient = Legacy_Auth, Srp, Win_Sspi

UserManager = Legacy_UserManager, Srp

WireCrypt = disabled

But that did not solve our problem. What else should we check? Telnet connection 'telnet remote_server_name 3050' was successful, so there are no firewal problems neither.

Thanks for any ideas in advance!