Given command:
"C:\Program Files\Firebird\Firebird_2_0\bin\gbak.exe" -T -B ":" "C:\Backup\BackupFileName.FBK" -v
the gbak 2.0.4 picks up some database for backup instead of giving error report.

Having this as a daily backup (due to batch error), it picked up different databases on different days...

Usual output is:
gbak: ERROR:file is not a valid database
gbak:Exiting before completion due to errors

Sometimes it writes:
gbak: ERROR:I/O error for file "MyDatabaseName
?"
gbak: ERROR: Error while trying to open file

(where the ? is char #04)

In cca 1/10 cases it had backed up some database...
It probably happens only, if another program has a database opened...? It also backed up the database after some time the last program, that had it opened, has terminated... (as detected from my logs)

Test batch:

@echo off

:retry

"C:\Program Files\Firebird\Firebird_2_0\bin\gbak.exe" -T -B ":" "C:\Backup\test.fbk" -v
if errorlevel 1 goto retry


During running this batch, other program starting or terminating sometimes triggers this...
Using -B "localhost:" also sometimes triggers this error...