postgres 7.1 security problem?

My postgres 7.1 now runs for several weeks without problems. Today I
suddenly got aware of the fact that no passwords are needed anymore to
login to any database.

Seems that the security system has been defeated on some way. pg_dumpall
-g still shows the correct users and passwords.

I don't know what went wrong here. This is a very severe situation for
me, so I would much appreciate any hint on how I could check the
security system and make it work again.

My postmaster gets started with the following command:

  su -l postgres -c "/usr/local/pgsql/bin/pg_ctl  -D $PGDATA -p
/usr/local/pgsql/bin/postmaster -o "-i" start >/dev/null 2>&1" <
/dev/null

Regards
--marcel