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I had the same problem and solved it. I had over 11.000 fonts in YourUsername > Library > Fonts. This seems to be the problem.
1. delete (or copy to your desktop) all fonts in YourUsername > Library > Fonts
2. clear the program and system caches. I did it with FontExplorer. As far as I know you can also do it with terminal commands.
3. restart your computer
4. add the font(s) to the Apple Font Book Software
5. start Powerpoint