On my desktop and my laptop - both with Windows 10's latest updates and ITunes 12.x - this problem has come back with a vengeance and may not be fully resolvable. It appears that Windows no longer allows Apple's USB drivers to install over their 2006 (!)
MTP USB device driver. You can no longer select the actual driver for the IPhone 6 at all. Reinstalling Apple's software only works as a temporary solution. Windows will eventually replace the driver with their 2006 driver. If anybody has found a way around
this, please post it.
I realize that there is little incentive for either company to resolve this as Apple wants people to buy their new product and Microsoft does not want people to buy anything from Apple.
I have this problem on 2 systems so I am pretty sure its not system or account specific.