There seems to be a bug in the PowerPoint 2007 v.2.0.0.0 file type, which manifests itself when the name of the .pptx file contains an URL encoded umlaut (%C3%BC) or possible any URL character encoding. I'm not sure if encodings are theoretically allowed in .pptx file names, but they do exists in the wild, and the main problem is that the error message is misleading: it indicates that the reason for the error is that the file is password protected ("This document could not be opened because it is password protected"). I've attached a screenshot of the error. You can reproduce this error by trying to add a file with an umlaut in the name into a Studio project (e.g. G%C3%BCnter.pptx).

Workaround is removing the URL encoded characters from the file name.