Need quick help with standalone mode

Hi!

I'm really in big trouble with a production database. It doesn't accept
connections anymore:

2007-07-31 19:27:33 CEST WARNING: database "userbase" must be
vacuumed within 999832 transactions
2007-07-31 19:27:33 CEST HINT: To avoid a database shutdown, execute
a full-database VACUUM in "userbase".
2007-07-31 19:27:33 CEST WARNING: database "userbase" must be
vacuumed within 999832 transactions
2007-07-31 19:27:33 CEST HINT: To avoid a database shutdown, execute
a full-database VACUUM in "userbase"...

So I tried this script (after shutting down the postmaster):

#!/bin/bash

/usr/local/pgsql81/bin/postgres -D /data/pgsql/data/lindau userbase << SQL
VACUUM FULL VERBOSE ANALYZE
SQL

But all I get is:

backend> 2007-07-31 19:27:33 CEST WARNING: database "userbase" must
be vacuumed within 999831 transactions
2007-07-31 19:27:33 CEST HINT: To avoid a database shutdown, execute
a full-database VACUUM in "userbase".
2007-07-31 19:27:33 CEST WARNING: database "userbase" must be
vacuumed within 999830 transactions
...
2007-07-31 19:27:33 CEST WARNING: database "userbase" must be
vacuumed within 999809 transactions
2007-07-31 19:27:33 CEST HINT: To avoid a database shutdown, execute
a full-database VACUUM in "userbase".
2007-07-31 19:27:33 CEST ERROR: could not access status of
transaction 539227074
2007-07-31 19:27:33 CEST DETAIL: could not open file "pg_clog/0202":
No such file or directory

I've provided the data path. So I don't understand the entry "could not
open file...". The database version is 8.1.5
and it runs on Redhat Linux 4 AS x86_64.

Any hints?

Thanks!
Robert