unable to back up iphone with error message 400
back up iphone failed with error message as abvove
Do you mean 4000?
If you see Error 4000 updating your iPhone with Windows iTunes - Apple Community user document. Click here to read --> https://discussions.apple.com/docs/DOC-250003773
See these 5 pages of discussion about iOS14 Error 4000 --> https://discussions.apple.com/thread/251804526
which was recommended by an Apple Community Specialist in this post --> https://discussions.apple.com/thread/252380437?answerId=254515885022#254515885022
Another solution by manually selecting the update file in WIndows:
https://discussions.apple.com/thread/252708257?answerId=255119643022#255119643022