I updated to Sierra yesterday and opened Nisus Pro today. The file I was working on loaded. I made some changes and the the automatic save feature told me that the file could not be found. "The file doesn't exist." The I got a message that the file server (i.e. the NAS drive where the file was stored) was shutting down.It did. I tried "Save as" with similar results. Then the program froze.
I didn't find the right solution from the Internet.