Symptoms
Connecting to FTP is failing and resulting in the following error:
421 Unable to read the indexed puredb file (or old format detected) - Try pure-pw mkdb=
Description
This error is caused by PureDB being enabled in the Pure-FTPd configuration which is not a default option enabled by cPanel.
Workaround
You will want to comment out or remove the PureDB line within /etc/pure-ftpd.conf as well as within /var/cpanel/conf/pureftpd/main (if it's configured within there as well) and then restart the FTP service:
/scripts/restartsrv_pureftpd