pg_dump: missing pg_database entry

Hello,

I'm experiencing a strange problem with PostgreSQL 7.4.9.
One of my database production servers has 2 large databases, it's still
possible to connect to them and pass queries, but the pg_database
system table is empty, which prohibits such actions as dumping the
databases.

For instance:

postgres@dial-bdd1:~$ pg_dump maf
pg_dump: missing pg_database entry for database "maf"

does not work,but:

postgres@dial-bdd1:~$ psql maf
Welcome to psql 7.4.9, the PostgreSQL interactive terminal.

Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help on internal slash commands
       \g or terminate with semicolon to execute query
       \q to quit

maf=#

This works, but the pg_database looks like it's empty:

maf=# select * from pg_database;
 datname | datdba | encoding | datistemplate | datallowconn |
datlastsysoid | datvacuumxid | datfrozenxid | datpath | datconfig |
datacl

---------+--------+----------+---------------+--------------+---------------+--------------+--------------+---------+-----------+--------
(0 rows)

I tried to repair the system indexes, but it doesn't work either...

Thanks in advance for your help.