MY FIX
I had a similar problem. There are many solutions, not anyone works for everyone. This is a Windows fix. I think many people here a confusing things, not indicating if their iTunes is running on Windows or MacOS.
Here is what I did:
In the windows file manager, under properties, for each of the files, in the iTunes programs directory.
AppleMobileDeviceHelper
ituneshelper
Synchserver
to xp compatibility mode.
Do NOT change compatibiliy mode on itunes.exe.
It only applies to Window Vista and newer.
It appears that apple does not play nice with Windows. You can either back rev to 10.3, which will correct the file compatibility, or you can fix them manually. They symptoms vary. When you’ve got incompatible code, all bets are off on what it will do.
There is a mistaken notion that the file permissions fix it. That’s not true. It seems that the apple code stops working from Microsoft Windows Vista and forward (7 et. al.)
All the other gyrations in the suggestions above are not necessary. I spent about six weeks on this one. My fix has worked fine now for two weeks, with daily synchs.
The relationship with Microsoft is tenuous, and fixes come late and last, if at all.
BTW, turn off “automatic updates” on iTunes, so you don’t get more bug-laden software from Apple.
Good luck.