Your best bet is to heed the advice PowerPoint gives you:
''"PowerPoint removed unreadable content in Presentation [Repaired]. You should review this presentation to determine whether any content was unexpectedly changed or removed."''
A possible cause is the following:
PowerPoint can insert content (such as pictures) into a presentation in two ways: either it ''embeds'' the content in the PP file itself, or it ''links'' to content external to the PP file. In the latter case, if the original content is deleted or its file path is changed, PP can no longer find it and might throw an error.
In order to fix this, open the PP file in Edit mode, and review for any missing content.