I had a problem this past weekend. I was upgrading a server from
FB1.0 to FB1.5.3. I originally installed it in classic mode but could
not connect to the server. I would get a 'failed to connect' error at
the client and in firebird.log I found the following message:

Database:
Unable to complete network request to host "COOKE-MTA".
Error while listening for an incoming connection.
An invalid argument was supplied.

Netstat showed firebird listening on 127.0.0.1:3050 but NOT
0.0.0.0:3050 as I would have expected.

I deinstalled classic and installed super server and everything worked
as expected.

The services file has gds_db 3050/tcp and the firebird.conf file was
left to default values.

Any suggestions?