A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
This was ultimately the fault of Protected View. Disabling Protected View allowed the files to be opened.
To disable Protected View:
File > Options > Trust Center > Protected View > uncheck all 3 boxes
This is, honestly, a terrible idea. Protected View exists for a good reason. However, disabling it fixes the problem, so it has to be done, at least until Microsoft deploys a patch.