That's how i configured it. Well, I mean, I can log into ftp using my original user, BUT, the access is restricted, so I KNOW that ISPConfig has it under control.
And thats the part were you are wrong. If you can login with your original Linux system user, then it is installed wrong and it is not controlled by ispconfig. If it would be installed correctly, you would get a login denied for your linux system user as the ispconfig setup uses virtual users and not system users.
Enable verbose logging in pureftpd and then check the syslog for error messages.