RESOLUTION
Examine /var/log/secure and /va/log/messages for any FTP server errors or login failures. If you see something like:Oct 17 17:03:32 proftpd[6084]: server_name.com (domain.com[192.168.0.1]) - USER user_name (Login failed): Limit access denies login Check /etc/proftpd.conf for Limit directives that can be a reason of the problem, for example the configuration below denies login for everyone except for users who belong to 'ftpusers' group:
<Limit LOGIN>
AllowGroup ftpusers DenyAll
</Limit> The access for Plesk FTP users can be granted by removing this rule or by adding 'psacln' group to AllowGroup. See ProFTPd documentation more information regarding Limit directive.
Keywords: invalid, user, password, ftp