Citation:
For migrating v.2.0.x or v.2.1.x databases to Firebird 2.5, a number of incompatibilities that are likely to affect existing databases or existing applications should be noted. It is not recommended that you begin a migration until you have resolved these.
Effects of Unicode Metadata
If you have not previously updated text objects within the metadata of your databases to be in character set UTF8, restoring a database until V.2.5 will fail with “malformed string” errors. To resolve this it is necessary to pay attention to the files in the /misc/upgrade/metadata directory of your installation and to use the new -fix_fss_data and -fix_fss_metadata switches in the gbak command line.