Q: the latest version iTunes cannot backup

after updated the latest version of iTunes 12.2.1 on 15 July 2015 ,i plug my iPhone 6Plus (ios 8.4) by using the original lightning cable ,until the end after finish running for syncing my phone and backing up (step 2 of 4) by using computer then iTunes show me "could not backup iPhone because iPhone disconnected" or "could not backup iphone because the backup corrupt or not compatible with iPhone (and ask me to delete backup for the iPhone and retry)".

i went Apple Store for help ,they recommant to uninstall the iTunes and download and install again , i had followed but until the end i still meet then same problem ,cannot backup .