I had this problem with Win81, PowerPoint 2013, pptx file, no system/software changes or updates that could be blamed. Out of the blue working on an existing document, messages as OP wrote started plaguing PPT. Brand new pptx files created with benign random
text also suffered from this. A save or two within the first 30 seconds of opening or creating the file sometimes went ok, but problem appears inevitably - implicating file lock issue.
There is remarkably little commonality in resolutions of this on the web, and no resolution from MS.
Tried many of the suggestions that can be found - no joy.
Finally isolated this to the live sync on DropBox. If DropBox is active (immediately syncing changes in the background), the problem persists. If DropBox is paused, the problem ceases. DropBox does not need to be removed or exited. If unpaused, problem
immediately starts again. Have not been able to check if other syncing cloud drives have same effect.
What's disturbing is that the DropBox/PowerPoint combination on this machine has been working fine for 1+ years. Turning off DropBox is not an acceptable solution. Perhaps a silent update of one or the other occurred to cause this.