Always use the latest release. For 2.5 it is 2.5.7. But even with this it is not
guaranteed to work. Check list of nbackup-related bugs in tracker:
http://tracker.firebirdsql.org/secure/IssueNavigator.jspa?sorter/field=updated&sorter/order=

I only see three items pending, but anyway, why not remove Nbackup from firebird until it is safe to use?

Why is there a tool that cannot be used in a safe way?