[till]

This is most likely a mysql connection problem so that the slave cant connect to the mysql on the amster anymore. This can e.g. happen when the login details are wrong or when a firewall was added on the master or when mysql on the master has been reconfigured to listen on localhost only.

You can test the connection by using the master login details from the file /usr/local/ispconfig/server/lib/config.inc.php from the slave server to connect to the master with the mysql command:

mysql -h master.server.tld -u ispcsrv1 -p dbispconfig