"iTunes could not back up the iPhone because the iPhone disconnected."

Every time I try to back up my iPhone 5s, I get this error message.

I have not been able to back up my phone in 6 months.

I have tried all steps on this article: https://support.apple.com/en-us/HT203843 ..., disconnected all other USB peripherals, etc. but the error persists.

iPhone 5s running iOS11.4, iMac running OS 10.13.4 (High Sierra).

Please help!