I'm not sure whether it's OSX or iTunes updates that have really mucked permissions up for iTunes. I also had a major issue. In the end having to create a new account & copying the files into iTunes to store into a new location.
I think it may be a folder permissions issue. Select the iTunes Music Library folder & see who is owner. Make sure the iTunes Music Library ownership is same as its parent folder & make sure it is owned by you or the system. I think it needs to be you not sure.

In my scenario I couldn't open iTunes at all, it would immediately come up with invalid location message or something similar & then close down.