I was told that it has to do with where your SNAP course is hosted. My understanding of the issue is this: If you save the files on a network drive (as we tend to do in our company), when you publish, you get this error because the network connection is slower than a local (c: drive) connection, and this slowness causes the publishing process to occasionally barf.
So the solution is to copy your .gpkg and .ppt files to somewhere your C:/ drive and try publishing from there.
This may sound a little wacky, but it has worked for me in the past.
Just to add a confusing detail, I have sometimes had success when publishing from a network drive- just not ALWAYS. I guess the best approach is if you get this error, try moving the file to the c: drive and then re-publishing and see if that clears it up.