VACUUM warning/error (?)

Hello.

We are running a small Intranet database, using PostgreSQL 7.0.2 on a
Debian GNU/Linux 2.2 server. All works fine. Great job PostgreSQL guys !

However, I used to backup our database every night (pg_dumpall), and
now, I want to clean up it just before the dump, when nobody uses it.
The command is :

# echo "VACUUM" | su -c "psql datavm" - postgres

...but the cron daemon just told me this morning :

NOTICE: FlushRelationBuffers(trace_sql, 162): block 239 is referenced
(private
0, global 1)
FATAL 1: VACUUM (vc_repair_frag): FlushRelationBuffers returned -2
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.
connection to server was lost

All seems to work, in particular this 'trace_sql' table, but what does
mean this error message ?

Thanks.
Niko.