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(at)dial-bdd1:~$ pg_dump maf
pg_dump: missing pg_database entry for database "maf"

does not work,but:

postgres(at)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.