I run on this problem the other day, I was getting a 530 Login authentication failed. I hadn’t changed anything on the account side nor had I updated the server.
I googled for possible solutions but most pointed to synchronizing passwords and double checking user names and passwords. So I connected with SSH and ran the script to sync passwords but it didn’t help.
After a couple of hours of frustration, I went directly to WHM to change the ftp from PureFTPd to ProFTP. When I logged in I was greeted by an different screen than usual. This new screen was telling me of some new available features in cPanel.
Apparently what had happened was that cPanel had updated automatically. This is fine but now FTP is broken. I figure the update must have caused some corruption of the passwords in the FTP server. Don’t know what or how.
So I figure I could try resetting the passwords on all domain accounts and see what happens. Well… to my surprise that fixed it!
I mean, it is not an ideal solution to have to reset your passwords but at least it worked.
If you know of a better solution please share it, because I would really like to know in case this happens again.
P.S. This was not the first cPanel Update, I have manually updated before with no problems. The version that cause this was WHM 11.36.0 (build 6)