If it does, you should ask yourself why it was missing in the first place. Note that this will only solve your problem if UTF8 is missing from RDB$CHARACTER_SETS. That will recreate the RDB$CHARACTER_SETS system table to include UTF8 again. The solution is to backup the database and restore it again.
If I manually delete UTF8 from this system table I get the same error when I try to connect with UTF8: SQL Message : -924īad parameters on attach or create database Harriv's comment prompted me to test my idea that this might be caused by a missing entry in RDB$CHARACTER_SETS.