BUG #2065: Problem with language conversion in pg_dump

The following bug has been logged online:

Bug reference: 2065
Logged by: Wim Audenaert
Email address: wima85(at)hotmail(dot)com
PostgreSQL version: 8.1.0.2
Operating system: Windows XP
Description: Problem with language conversion in pg_dump
Details:

I have a database created with encoding UTF8 while I used SQL_ASCII for the
installation of Postgres 8.1.0.

When I try to do a pg_dump of my database, I get the following error
message:

pg_dump: SQL command failed
pg_dump: Error message from server: ERROR: cache lookup failed for function
170
39
pg_dump: The command was: SELECT tableoid, oid, adnum,
pg_catalog.pg_get_expr(ad
bin, adrelid) AS adsrc FROM pg_catalog.pg_attrdef WHERE adrelid =
'23268'::pg_ca
talog.oid

I looked for the function 17039 in the pg_proc table, and this function
doesn't exist. It does exists in an older version of the database (8.0).

I tried to use the encoding parameter in pg_dump, but without success.

What can I do resolve this problem?