gbak: creating indexes
gbak: ERROR: invalid request BLR at offset 2129
gbak: ERROR: BLR syntax error: expected invalid BLR code at offset 2129, encountered 181
gbak: Exiting before completion due to errors

How to repeat:
Backup (with gbak from version 1.5) only metadata (-m option), e.g.
/opt/firebird/bin/gbak1.5 -m -g -l -b -k -e -v -t -user sysdba -password $YourPasswd /source/database.gdb /backup/database.bck
Restore newly created backup file:
/opt/firebird/bin/gbak -r -c -g -l -o -n -one_at_a_time -verbose -user sysdba -password $YourPasswd /backup/database.bck /databases/database.gdb

How to solve:
Open newly created database with IBExpert. Open context menu at Procedures and choose Deactivate procedures -> Deactivate All Stored Procedures. The same thing concerns (context menu on Triggers and choose Deactivate triggers -> Deactivate All Triggers).
Make backup of this database and restore with tools from version 1.5 of Firebird. Open it with IBExpert and the same way you deactivated try to activate your procedures and triggers. Uncompatible structures are to be revealed.

If the problem still occures comment all procedures, backup database together with data and restore it. When restore is ready activate all procedures.