I work with Firebird for more than 10 years and I have great experience with its administration and maintenance. Funny thing, my very first task as a Firebird developer was fixing a problem with unrestorable Firebird 1.0 backup.
Concerning your database, it seems that the error is shown during restore of some procedure using the SIMILAR TO predicate. Although such a syntax is available since Firebird 2.5, and you're using Firebird 2.1. Either you're trying to "downgrade" your database from 2.5 to 2.1, or a database in 2.1 format was worked with by 2.5 server.
Possible solutions are: either rewrite procedures to avoid using of SIMILAR TO, or start to use Firebird 2.5. The 2.1 version is discontinued since 2014, after all.