When attempting to connect to SFTP the following errors are generated:
Error: FATAL ERROR: Received unexpected end-of-file from SFTP server
Error: Could not connect to server
In this instance, this issue was caused due to the "sftp subsystem" line missing from the SSH configuration file at /etc/ssh/sshd_config.
Adding the sftp subsystem line to the SSH configuration file at /etc/ssh/sshd_config and restarting the service should restore access to SFTP:
Subsystem sftp /usr/libexec/openssh/sftp-server