The first link will help you fix any possible permissions error that prevents iTunes from reading or saving the library file.
In the second link the relevant section is this one:
In a similar vein it is possible that a corrupt preference file may cause iTunes to fail at launch with one of these errors:
- The file iTunes Library.itl cannot be read because it was created by a newer version of iTunes.
- The folder “iTunes” is on a locked disk or you do not have write permissions for this folder.
- The iTunes Library.itl cannot be found or created. The default location for this file is in the 'iTunes' folder in the 'Music' folder.
It may be worth trying the shift-start-iTunes method (option on a Mac) to actively select the .itl file that should contain your library. If you still get the error message try Repair security permissions for iTunes for Windows and/or check for interference from anti-virus software.
The last link includes a section on completely removing and reinstalling iTunes, which may workaround some error conditions.
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