Yes, this has happened to me. My solution:
Uninstall the Package Editor, Uninstall VS, reinstall VS, reinstall the package editor. AFAIK there is no other solution.
IIRC, this occurs when you install Office after installing VS on a machine, and Office changes the registry or something in some way that causes this behavior.
Things like this is why I always have two fully functional development machines running – I can switch to the other machine, and continue working while uninstalling/reinstalling.
Wish I had a better answer for you, but I’m afraid that this is the only way to deal with this.
You might be able to do File/Open the PPTX file and it might open using the Package Editor, even if the registry is messed up. It has been a couple of years since I encountered this because I am careful to first install Office, and then install VS, and finally install the Package Editor.
Cheers, Eric