I noticed an alarm in vCenter that indicated the vCenter Server Appliance (VCSA) was having trouble backing up.  I logged into the VCSA Appliance Management interface (vCenter URL with :5480) to investigate.  When I logged in, I found the following error:

Failed to create backup directory on Backup Server

Thinking there was something wrong on my SFTP server, I checked the username and password, file system permissions; but could find nothing wrong. I connected to the VCSA using root on SSH, and checked the log file at /var/log/vmware/applmgmt/backup.log.  This provided some additional detail on the issue and showed the backup process was unable to authenticate to the SFTP server.

I tried the SFTP command manually and found the core of the issue:

For some reason, the SSH key has changed and needs to be updated in the local SSH Known Hosts file.  Once I accepted this new key, the backup was able to complete successfully.

I am not sure what would have changed the SSH key.  The only thing I can think of is that I had updated vCenter from 7.0 U3f to 7.0 U3g recently.  I have verified the host is correct.