Last tested: Aug 18, 2020
This occurs when information about the host being connected to via SFTP doesn't match the saved information about the host on the instance. Via the command line this results in a warning that can be bypassed manually and transfers work fine. Looker does not do this however, and will instead throw a "Host key mismatch" error.
To get around this, we need to drop the existing info about the host from the instance so the new info will be populated into the known hosts directory.
If you host your own instance, on the instance run
cd .ssh and then
ssh-keygen -R hostname (where hostname is the name/IP of the SFTP server). From there, simply run another SFTP transfer via Looker, this will add the new info about the host to the known hosts directory and remove the error. If your Looker deployment is hosted by Looker, please open a Looker support request for assistance.
This can happen if the SFTP is restarted and the destination IP address has been changed (even though it's the same host name). So the known host entry is now wrong and the SFTP process thinks its a man in the middle attack.