recovering fs-data from previous installation

I managed to install again postgresql-8.4 from hardy-backports.
Now the installation is identical with the previous one.

With the fresh database, the server starts up fine.

Now I copied all from the backup to
/usr/lib/postgresql/8.4
/var/lib/postgresql/8.4

The the startup fails like this:

* Starting PostgreSQL 8.4 database server * Error: could not exec /usr/lib/postgresql/8.4/bin/pg_ctl /usr/lib/postgresql/8.4/bin/pg_ctl start -D /var/lib/postgresql/8.4/main -l /var/log/postgresql/postgresql-8.4-main.log -s -o -c config_file="/etc/postgresql/8.4/main/postgresql.conf" :

the log-file is empty.

Can you please help to get postgresql to start again using the previous database.
I suppose there should be no serious issues, as the version of postgresql is identical now.