What is the exact error message, not only the error code?
"#1045 El servidor MySQL no autorizó su ingreso" (spanish)
Did you maybe restrict the user to allow access from loopback, only but configured mysql to access the public IP?
mysql is on default config (or the config that ispconfig3 installer modified by itself)
Let me clarify:
I created a client "myclient"
I created a site "mysite" for that client and then a database "mysitedb" for that site, then i created a user "dbadmin" for "mysitedb", when i try to log him over phpmyadmin to access his db (mysitedb) i got that error.
Besides, if i log in phpmyadmin with root / rootpass does work (and i see mysitedb), if i test the user "dbadmin" in command line, works fine against mysql and i can access "mysitedb", the problem is when i try to log with that user over phpmyadmin to access its database.