I have this same error when I try to save after doing a lot of inking in PowerPoint 2013. The file was stored in Dropbox, then I copied it to my local drive. I was editing the local copy (no one else was using it), and was able
to save several times. Later, after a lot more inking, I try to save, and I get the error about it being unstable, and that I should save and restart Powerpoint. But if I try to save, I get the error again. I tried "save as" to save it to a new filename (again
on my local drive), but I get the same error.
I can save as a .ppt instead of .pptx, but then some formatting, etc., is lost.
This is not just a minor inconvenience -- it represents a lot of work that is lost. I have found PowerPoint 2013 to be generally unstable, sometimes crashing every 15-20 minutes while creating a presentation. It has not crashed while showing a presentation.
I am using a Surface Pro.
I am using a Surface Pro, also and am having the same issue.