Please check that you used the full ftp username incl. prefix as it gets displayed in the FTP user list in ispconfig (not the details page).
If thats ok, check if the mysql username and password that you find in the pure-ftpd sql config gile are correct (e.g. by entering them into phpmyadmin).
Then check if the ftp user directory exists ( /var/www/yourdomain.tld/ ).