I also had same issue
Centos 6.8
Apache version: Apache/2.4.25
PHP version: 5.6.30
MySQL version: 10.1.22-MariaDB
FTP version: 1.0.30
You try to do this
nano /etc/pure-ftpd/pure-ftpd.conf
Comment out this line (Added # at begining of the line)
#PAMAuthentication yes
Enable UNIX authentication
UnixAuthentication yes
Restart
service pure-ftpd restart
Then you try to change the password of the user in CWP (on browser)