Hello. Thanks for your answers.
Yes, I know this change for Firebird 2.x (IPC vs XNET).
With Firebird 2.x it works good, with string connection <servername>:<server path to DB>. Clients, in the LAN, can connect without problem using port 3050 (firewall opened), using the same string.

I uninstall 2.x and install 1.5.6 and the same application cann't connect (with the same connection string).

Using port 3050 (opened on firewall). "Firebird.conf" file parameters reviewed.
Using connect string (I always use this pattern):
<server name>:<local server path to DB>

I used gsec in the same server (on a cmd window). As Helen says, I'll test gsec with "-database". But "gsec" was only a test. My problem is with client connections.

Can I check something more?

Thanks.