Incorrect "not enough free space to backup iPhone" error
My 256GB iPhone X has only 54GB of space being used. My MacBookPro has over 200GB of free space. And yet iTunes is telling me there isn't enough space to back it up. I've deleted all previous backups just in case. I've never managed to back up this iPhone since I got it in December, so I can't rule out an issue with the phone itself, but that seems unlikely.
Other posts about this issue over the past year have not been resolved, or have been "solved" with a link to this (unhelpful) article:
If your iTunes backup couldn't be completed or you can't restore from a backup
I've tried restarting both the iPhone and the MBP. Both are running the latest OS. I can live with only iCloud backups, I suppose, but I've always done periodic backups to my Mac via iTunes for extra safety.
Any suggestions on how to solve this would be most welcome.
MacBook Pro TouchBar and Touch ID, macOS High Sierra (10.13.2)