Are there any errors logged when attempting to login using one of the newly-created FTP accounts?
Code:
# tail -fvn0 /var/log/messages
When creating or removing FTP accounts, are any messages logged by cPanel?
Code:
# tail -fvn0 /usr/local/cpanel/logs/error_log
For possible fixes, I would try using the following feature in WHM to synchronize FTP account passwords.
WHM: Main >> cPanel >> Synchronize FTP Passwords
Here is the equivalent command to do the same via root SSH access:
Code:
# /scripts/ftpupdate