Error on VACUUM

Hi,

I am migrating from 6.2 to 6.4.2 a database and before dumping I
made a vacuum in the 6.2  I have got the following error:

mydb=> vacuum;
NOTICE:  BlowawayRelationBuffers(logins, 26): block 40 is referenced
(private 0,
 last 0, global 1)
FATAL 1:  VACUUM (vc_rpfheap): BlowawayRelationBuffers returned -2
mydb=> \q

I have alreade made the dump, but I tried to remove the pg_vlock and
tried again another vaccum and I get the same error.

Anyone knows if this error implies loosing  data?
(The restored DB makes the vacuum without problems. )

Another question simple question, how can I change the name of
a Database?

Thanks

Giovanni Floridia