Indicates that shell isn’t listed in /etc/shells, and ProFTPd is making sure the user is allowed to login based on whether the user has a valid shell (and, thus, is not a system account or some sort of non-interactive user that shouldn’t be logging in).
I’m pretty sure we add /sbin/jk_chrootsh to /etc/shells during installation, but maybe it got replaced in a system update or similar. Try adding it and see if the login works.
They’ve definitely turned on Jailkit, but that shouldn’t prevent ProFTPd logins from working. ProFTPd is not subject to the jail (it doesn’t run a shell, but it’s verifying the user is an actual interactive user allowed to login via FTP by checking the shell).
Yes, when you enable jailkit for a user, their shell must be jk_chrootsh. If you changed the shell to some other interactive shell (like /bin/sh or /bin/bash), you disabled the jail for that user and their ssh sessions will not be jailed (though ProFTPd can restrict a user to their home without jailkit or a chroot).
Just adding jk_chrootsh to /etc/shells should allow users with that shell to log in to ProFTPd.