Error in postgres server

I got the following error repeatedly from my postgres (6.3) server:

NOTICE: SIAssignBackendID: discarding tag 2147422478
FATAL 1: Backend cache invalidation initialization failed

The database houses substantial amount of information but not too much,
and is accessed via the psql C interface and ODBC. I try to vacuum every
night, but one DB is not vacuumed that often. Any ideas what could be
wrong? After rebooting, the problem disappeared.

Thanks in advance for any pointers.

Paul.