Check the path of files CONTROLLER.FDB and TARIFACAO.FDB in file "aliases.conf" (by default: C:\Program Files\Firebird\Firebird_1_5).
Check the path of the database in the billing machine in Configuration > Database, and the address 127.0.0.1 must be configured if the database is installed in the same computer. If the Database is installed in another PC of the network, place the IP address of the computer where the database is installed.
Check how many Firebirds are installed in the machine using the Controller.
Controller 1.0 is only compatible with Firebird 1.5. Controllers Pro and Corporate are compatible with Firebird 1.5, 2.0 and 2.1. In this case copy file aliases.conf of the folder of Firebird 1.5, and paste it in the folder of the other version.
Deactivate Firewall and antivirus of the computer.
Check if the two processes fbguard and fbserver are being executed in the task manager. As a last resort, reinstall the Controller.