I could be wrong but I think you would not use the host/domain for the connection to the MySQL server but instead, since your scripts are on the same box as the MySQL server, that you would instead use either localhost
is a safe bet to work.
But, if you have phpMyAdmin installed and it does actually work, you could do what I do and that is cheat, take a look at phpMyAdmin's configuration file which should be, if installed as a package through ISPConfig, in
Take a look in there and see what it has listed for the "$hostname" because I think it might work better with either :
$hostname = "127.0.0.1";
$hostname = "localhost";
That's just a guess though.