Today I ran into the “another sync client is already running” and “session could not be started” errors whilst trying to sync my iPad 1 which is using iOS 4.3.2 running iTunes 10.3.1 (55) on Mac OS.
These are the steps I performed that fixed my problem(s) syncing after reading Apple forums solutions with no success and then experimenting with several techniques before finally finding one that worked.
First, reboot your iOS device, and do the same for your desktop machine. Connect your iPad (or iPhone) to the desktop computer and start iTunes if it does not automatically open after you connect your device. In iTunes under the Devices section at left, select the iPad ensuring it is highlighted, then Ctrl-Click (Mac) or “right click” (Windows) the iPad or iPhone. In the context menu that appears, click the Reset Warnings option, then click Sync. You will now see any problems that occur during synchronisation, and iTunes will fail once again, however this time you should know the cause.
I discovered that iTunes did not like three movies a podcast had placed in iTunes, a warning message fingered the culprits by revealing the names of the movies that are unable to play on my iPad – this is the root cause of the error that caused attempts at syncing to fail. On your system you may see errors related to movies, music files, podcasts, books or photos. Because the warning message pertained to some of my movie content, I clicked the Movies tab at top, then unticked the automatically include all movies and unticked each movie in the Movies box.
After performing a similar action based on the warning message your system gives you, you may now click the Apply or Sync button located at the bottom right of the iTunes window. After clicking the Apply button and syncing successfully, I removed the problem movies from iTunes, and restored my old settings by ticking (checkmarking) the Automatically include all Movies box and clicked Apply button, thereby syncing all movies with success to my iPad.
Please note others have tried other techniques such as removing old backup files with various success. Backup file removal did not work for me although it may on your system. Other mentions of this issue on the Apple support forums are found here and here.