Update: 5/24/16
There is a new iSyncr Desktop for Mac users with iTunes 12.4 support. If you are using a Mac and iTunes 12.4+, please make sure you are using iSyncr Desktop 5.1.10 or higher. There are no known Windows issues with iSyncr Desktop and iTunes 12.4 at this time.

Update: 5/20/16
Sharing the iTunesMusicLibrary.XML has been turned off by default in iTunes 12.4. This setting is found by going into iTunes, selecting preferences, advanced then share iTunes music library.xml.
iTunes 12.4 has changed the location IDs to your specific songs in iTunes. Any old play count information that is stored on your Android will not sync resulting in a timeout over WiFi followed with an error code 6. Or you won't be able to update ratings/play counts to iTunes.

To fix play count and rating update issues, delete the iSyncr.xml and DeltaRatings.xml files with the old iTunes IDs from the /syncr folder of your Android device. Then re-sync everything (not recopy everything) on your device, and allow iSyncr/Rocket Player to get the new IDs. Do this by accessing the /syncr folder with a file management application on Android, then deleting the iSyncr.xml and DeltaRatings.xml file.
Further information will be added to this blog post when it's available.