> I don't have any problem inserting or reading greek characters from the database, so I presume that the connection does allow Greek characters (WIN 1253)

Your presumption is wrong. Character set NONE just don't care about
kind of characters you are using. It is working as long as your fields
are also defined with character set NONE. But for triggers it is not
true - their bodies (in FB2.1 and above) are defined with character set
UTF8.
Firebird have no idea how to convert NONE to UTF8 thus the error.

SY, SD.