Many promised fixes online, but this is the one that works. Just leaving this solution here for posterity: https://discussions.apple.com/thread/4410910
“What I did was to rename the file called conflicts.synconflicts I renamed it to conflictsold.synconflicts the .synconflicts is the extension so you can’t rename that. I then restarted my computer, opened iTunes and connected my iPhone.
OS(C:) Users/(USER NAME)/AppData/Roaming/Apple Computer/SyncServices/Local/conflicts/conflicts.synconflict”
Actually you don’t even need to restart.
Update 2023: There are now two files related to this issue, which are now found in the AppData/Local/Packages/AppleInc… folder. Just delete them both.