Hi,
just for your information‌, I had a problem to backup my S6 and Mini4 (both iOS 10.3.3) via USB to the latest version of iTunes. Sync did work, but not Backup. Same on iMazing. The iCloud backup worked, though. On iTunes the devices were disconnected during the backup process; it`s the same Error, that affects iOS for some years, coming and going randomly.
It‌ showed via special Log-Files on iMazing, that one of six locally stored Flightplans caused the abort and loop on both programs. Nothing special although on the file concerned, just a plan from Budweis to Hohenems. Deleting it and the backup worked, no need for any special workarounds circling in the Net on this issue.

Harald‌‌