Strange query results with invalid multibyte character

Hi,

I have a strange problem.

I have 2 database, both created with:
CREATE DATABASE muntdev WITH OWNER = postgres ENCODING = 'UTF8' TABLESPACE = pg_default;

Running the queries below on the first database is OK and do what I expect.

If I create a backup of the first datase and restore that backup in the second database the following happens:
The first query (see below) returns 17 records, all containing 'Boonk%'.
The second query (see below)returns 'ERROR:  invalid multibyte character for locale'

Query 1:
select lastname from salesordercustomer where lower(lastname) like 'boonk%'

Query 2:
select lastname from salesordercustomer where upper(lastname) like 'BOONK%'

Both database are running *exactly* the same version of Debian Etch AMD64 and *exactly* the same version of PostgreSQL
(8.1.4for AMD64) .... 

Can anyone explain this to me?

TIA

Joost