This is not a common issue but once it happens and if don't know the correct fix, it is going to screw your day.
Issue: You will not be able to login to FTP accounts of the domain. It will throw "Login incorrect error", eventhough the logins provided are correct. We faced this issue recently and the following steps was performed to fix the issue.
First of all check which FTP is using the Cpanel, proftpd or pureftpd
If it is using the proftpd, do the following to fix the 530 error
1. First of all try the /scripts/ftpupdate
2. grep <username> /etc/proftpd/passwd.vhosts (here <username> is the domain username)
3. If it is not present there then grep <username> /etc/proftpd/<username>
4. Check details and cat >> /etc/proftpd/passwd.vhosts
5. Paste the main ftp account details in this file (eg.hipingla:$1$gWMrNEGZ$IZyVQmx.1I/EBNS585cO3.:33203:33206::/home/hipingla:/bin/ftpsh)
Note: please be careful in doing it. If you have specified cat > /etc/proftpd/passwd.vhosts, it will overwrite the whole content.
If it is pureftpd:
Do the above 6 steps
7. Sometimes you need to run "/usr/sbin/pure-uploadscript -B -r /usr/local/bin/ftpfilter" also