Fixed it!
I created a new Unraid USB stick to compare the contents and found the only obvious thing that was different was that I was missing a "dynamix" folder under /config/plugins. I had a file called "dynamix", but not a folder; something must have gotten corrupted somehow. Anyway, I deleted the file, copied the folder over and everything is working as expected again.
So the server image and the notification settings are all stored in the /config/plugins/dynamix folder for anyone who stumbles across this in the future
