Thanks Till for getting back to me.
I figured it out.
When the second server is installed, it changes the ispconfig database user password.
as the mysql database is on the shared storage, this is reflected on both servers.
Now the password for the master server is wrong.
I think the install.php should find a way of updating /usr/local/ispconfig/server/config.inc.php with the new password for the ispconfig user, or create another user (let's say ispconfig1) for the second server.
Thanks anyway for taking the time to respond