Cannot connect although server is running

I posted this to comp.databases.postgresql before but got no answers yet.
Traffic seems to be pretty low over there. So here

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Since about a month I can't connect to my local postgres server
any more.

$ psql
psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/tmp/.s.PGSQL.5432"?


However, the server seems to be running fine:

$ ps aux |grep postgresql
postgres 11406  0.0  0.2  21812  3960 ?        S    06:24   0:00 /usr/lib/
postgresql/8.1/bin/postmaster -D /var/lib/postgresql/8.1/main -c
config_file=/etc/postgresql/8.1/main/postgresql.conf


$ sudo /etc/init.d/postgresql-8.1 status
8.1     main      5432 online postgres /var/lib/postgresql/8.1/
main       /var/log/postgresql/postgresql-8.1-main.log

In /etc/postgresql/8.1/main/pg_hba.conf I granted access to any local
user:
local   all         all           trust

System is debian/squeeze

Any hints?
Maybe some kernel options set the wrong way? But the server does not work
connect with a vanilla 2.6 kernel from my distro, either. So I conclude, the
problem must be elsewhere.

---<(kaimartin)>---