pg_dump: schema with OID 1 does not exist

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Hi there.
We have a big problem with one of our databases.
Postgres Server 7.3

When we try to backup the database, pg_dump exits with the following error:
pg_dump: schema with OID 1 does not exist
The same issue appears when we copy the entire database and try it again.

All the data are stored in the public schema.

Thanks a lot for helping me!

Christian
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