iTunes usually keeps two files with information on your library: "iTunes Library.itl" and "iTunes Library.xml"
The *.itl-File is solely used by iTunes and in an undocumented, obfuscated format, whereas the *.xml-File can be opened up with any text editor, and has a pretty self-explanatory structure to it.
These files usually mirror most of the same information, but there's some iTunes-specific information, such as the "Date Added"-Tag of Files (which tracks when a song was added not to Disk but rather to the iTunes Library), or playback information (e.g. in iTunes it's possible to have songs start and stop individually at defined times) solely stored in the *.itl-File.
Now I'm taking a wild guess here, but it appears that something in this last release of iTunes is causing turmoil within this information (or other similar information that is only used / read by iTunes such as the "Crossfade Songs" information or Gapless Playback), thus causing playback issues. In my case it only affects songs that have been imported recently, and not older ones that have been in my library for years now, these play perfectly fine.
The solution I've come up with currently involves this:
- Checking whether in iTunes -> Preferences -> Advanced the "Share iTunes Library XML with other applications" setting is checked / turned ON.
- Closing iTunes.
- Opening up the iTunes Media Folder Location in Finder / Explorer.
- Deleting (or to be able to revert it, just in case, renaming) the "iTunes Library.itl" while making sure to keep the *.xml-file.
- Creating a new Library for iTunes by launching it with ALT + *Click on Dock Item*.
- Importing the *.xml-file to this newly created, empty library via File -> Library -> Import Playlist
Afterwards it's gonna take a couple of minutes, depending on the size of the library you might wanna make a coffee, and then all playlists & songs should re-appear.
Unfortunately this does mean some changes / inconsistencies might happen to your library:
- Date Added & further iTunes-related information is lost fully.
- Some files might get lost during iTunes' reorganisation process.
- Not too sure whether or not Smart Playlists are completely transferred.
Here's to hoping Apple acknowledges this bug and fixes it soon (: