petka82

Thanks,

The HyperDB configuration does not say whether I should copy the configuration files on the master only or onto slaves as well.

What about the

read_only=1

in my my.cnf, shall I keep it on slaves or not?

Check your DB credentials in the wp-confg-db.php if they are alright.
The best would be if you tried to do

mysql -u youruser -p

type in the password and check if all works.