Thats most likely a problem with mysql permissions. The slave server is not able to connect to the master, so he can not update the monitor.
Have you entered the hostname of the master mysql server while you setup the slave in the ispconfig installer as requested and not the IP of the master? If you entered the IP, then the connection will fail. Another possible problem is if you did not alter the /etc/hosts files on both servers as described in the multiserver guide, the connection will only work if both servers can resolve their hostnames in the same way.