pg_dump problem

Hello,

    I've got a problem during invoking the pg_dump program: I
get the following result:

NOTICE: get_groname: group 2 not found
getTables(): SELECT failed. Explanation from backend: 'pqReadData() --
backend closed the channel unexptectedly

The point is that everything used to work fine, and I don't know actually
when and why this error ocurred.

I did the vacuum a few times but it didn't help. The error occurs also,
when I try to dump a single table - even if it has just been created as a
new one.

What can be the reason and what should I do to make a backup of the
database?

                    thanks in advance

                        mark