unable to back up iphone with error message 400
back up iphone failed with error message as abvove
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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