Symptom:
Error "database unavailable" when trying to connect to a Firebird database even if fbserver is running fine (and Java applications connect, isql works great, remote clients can connect great, etc.).
Scenario:
You have just installed Firebird on a machine that previously had Interbase installed and are trying to run a Windows native application to connect to the database.
Solution:
Interbase installs GBD32.DLL into the Windows/System32 directory. When upgrading/moving to Firebird an option is given to create a legacy DLL for Interbase clients to connect to Firebird. This DLL must be manually copied over the existing GDB32.DLL from the bin directory of the Firebird installation.
Close the client application first (this can be Delphi for example if you're still writing it and get the error during development).