problem with table system

When i try the command \dt or when i try to extract directly from the
system tables , i've the following error

docserv=> \dt
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.

Vacuum don't work :
docserv=> vacuum verbose analyze ;
NOTICE: CreatePortal: portal <vacuum> already exists
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.
We have lost the connection to the backend, so further processing is
impossible. Terminati

Extract of postgres.log

StartTransactionCommand
query: SELECT usename, relname, relkind, relhasrules FROM pg_class,
pg_user WHERE usesysid = relowner and (
relkind = 'r' OR relkind = 'i' OR relkind = 'S') and relname !~ '^pg_'
and (relkind != 'i' OR relname !~ '^x
inx') ORDER BY relname
ProcessQuery
/usr/lib/postgresql/bin/postmaster: reaping dead processes...
/usr/lib/postgresql/bin/postmaster: CleanupProc: pid 25962 exited with
status 11
/usr/lib/postgresql/bin/postmaster: CleanupProc: reinitializing shared
memory and semaphores
shmem_exit(0) [#0]
binding ShmemCreate(key=52e389, size=2006016)

Thanks , Loic