I've seen similar issues like mine from 2010 and recently in a post that mentioned a specific add-on. The fixes for the 2010 issue do not work for me and I am not using the add on that was mentioned in the more recent posts.
Here is what I am experiencing. I'll have an Excel worksheet open (does not seem to be occurring on the same spreadsheet each time, this has happened on every excel file that I have had open over the last few days) and while I am working a message will pop
up that warns me that autosave has been turned off for this session. Since this is an obvious sign that something isn't quite right, I close down the files that I have open so that I can restart Excel. When Excel asks if I want to save the files, I say yes
and then receive this message:

Clicking "Continue" gives me the option to save the file (I've tried renaming the file, saving it to my local drive, network and OneDrive) but then gives me the error message "Document not saved" when I click "Save".
Prior to the message telling me that Autosave has been turned off, I am able to save my files without any issue.
I do not use other Office applications enough to know if this is occurring with those files.
- Excel 2016 MSO (16.0.6701.1036) 64-bit
- Windows 10 Pro 64-bit