I updated to CentOS 7.6 and something must have changed in the base OS setup that prevents vsftpd from allowing logins for accounts with /sbin/nologin as their shell. I had to add that to /etc/shells so that such accounts could FTP again. That file is in the setup package. Did it include /sbin/nologin before? I don't have anything in my notes from setting up the system last year about changing that. What's the history of shells listed in /etc/shells?