I still see the default line, which is:
$rcmail_config['db_dsnw'] = 'mysql://roundcube: pass@localhost/roundcubemail';
You should change that into:
"web1_u1" is the mysqluser for the website (in this example =web1).
"mysqlpassword" is the password for mysql user web1_u1.
"web1_db1" is the databasename for web1.
All as defined within ISPConfig under the "Options" tab, where you can define the database for your website.
| Master in managed hosting