The problem seems to stem from a defective "iTunes Library" file and 
rebuilding it has solved the problems for that user.

WARNING: 
Follow the instructions fully at your own risk. Smart Playlists can 
possibly be turned into regular ones. I haven't tried out the solution 
myself, yet, though. Making a copy of your iTunes Library and iTunes 
media files certainly can't hurt.

Instructions to rebuild your iTunes Library:

1) Quit iTunes.

2) Go to ~/Music/iTunes and rename "iTunes Library" to f.ex. "iTunes 
Library (Old)"

3) Start iTunes.

4) Choose "Import Library" from the File menu.

5) In the dialog now opening, choose the file "iTunes Music 
Library.xml" located in ~/Music/iTunes/

6) Wait for iTunes to complete rebuilding your library cleanly from 
scratch.

7) Restart iTunes.

If you try out this solution, please share the result here. Thanks!

Best regards, 
Felix Schwarz