Some hubs cannot supply the minimum 0.5 amps that the iPhone requires to sync. This is true of all hubs that don't have an external power supply, and some that do if it isn't big enough. USB ports on the front of most computers are an internal, unpowered hub.
Just found your post and possible solution............... Worked for me also! I unchecked the "Sync with this iPhone over Wi-Fi" on the main iTunes sync page, clicked "Apply", and my iPhone successfully backed up after a couple of weeks of no luck.