iPhone backup to MacBook Pro is not completed

Dear Apple Community,

I would like to create a backup of my wife's iPhone on her MBPro (backup on the device, not in the cloud), but the backup does not finish. It slows down extremely at an estimated 99% and the data is only written KB by KB. I have let it run overnight for more than 12 hours and tried several times, but the backup simply won't complete.

I have even redirected the backup to an external HDD with 500GB. But the result is the same.

iPhone XS with iOS 16.7

MBPro with macOS 15.5 Sequoia

Do you have any idea what the problem could be?

Thank you and best regards!

PS: Cable is original Apple and brand new; the backups were always cleaned up after unsuccessful attempts or the uncompleted ones were deleted; there is a constant Internet connection via LAN etc.; there is enough space available...