As someone mentioned in the thread, this is not an issue of PostgreSQL.
I am not at all familiar with the details of pgAdmin, but I surmise that the error message indicates that the characters you entered, or the ones sent by PostgreSQL, do not match the encoding that pgAdmin expects. It would be worthwhile to ensure that the language settings of pgAdmin are consistent with the connected database and local environment. For further assistance, you should consult the pgAdmin team.
regards.
-- Kyotaro Horiguchi NTT Open Source Software Center