Notice that the database MUST be placed on the same machine, you have installed the Firebird server.
(eg. if your Firebird server is placed on your local machine, the database should be placed at the same local machine).

If you installed the Firebird server without marking the box at “Copy Firebird client library to <system> directory”, at step 8 in the extended installation guide , as shown below, this will help you overcome the problem, without reinstall the Firebird.

If you open your command prompt (cmd) and navigate it to your firebird installation directory:
C: \ Program Files (x86) \ Firebird \ Firebird_2_5 \ bin
(or where your Firebird is installed)

For our example, the following command is used:
CD C: \ Program Files (x86) \ Firebird \ Firebird_2_5 \ bin

Next, type two commands:
Instclient.exe i fbclient
and
Instclient.exe i gds32

Then it will help you get it all right and installed and then you should be ready to go.