OK, I found the answer, well an answer.
Searching in the syslog translator application itself didn't show me anything, but going via a Knowledge Base entry did, see the link in this article: https://kb.netapp.com/app/answers/answer_view/a_id/1030291
Apparently : This message occurs when the host keys on Secure Shell(SSH) server is NULL due to some corruption or loading of host keys has failed.
Corrective Action is: Disable ssh and then enable ssh as follows: options ssh.enable off options ssh.enable on
Unfortunately it's not really that clear exactly which file(s) are corrupted/missing. I can ssh into the Filer apparently without issue.