Update to iOS 14.4 failed - "The iPhone could not be updated. Error code 4000"

I have an iPhone 11, on iOS 14.3. iTunes is 12.10.10.2. The update to iOS 14.4 progresses through 'Preparing iPhone for software update', then halts with "The iPhone...cannot be updated. An unknown error occurred. (4000)". 'More info' does not list Error code 4000.

iTunes is updated. Computer software is updated. I use direct connections, no WiFi, for security.

I tried every suggestion posted on these help pages. Five hours!

I turned off iPhone and computer and unplugged the iPhone. I plugged the lightning cable into different USB ports. I turned on the iPhone and computer in different order. I rightclicked the iPhone icon > Properties > Shortcut tab, Advanced > Run as Administrator > OK > Compatibility Tab > Run this program in Compatibility Mode for Windows 7 (and also tried 'Vista') > Change settings for all users > OK.

Clicked on iTunes icon (and tried rightclicking and Run as Administrator) > Yes > click on the vertical-rectangle icon for iPhone > click on the iTunes popup "A new iPhone software version (14.4) is available for the iPhone. Would you like to update your iPhone now? > Update. (Also tried clicking on the non-popup 'Update'). Every time the update progresses through "iPhone...cannot be updated. An unknown error occurred. (4000)." I repeated with Norton 360 and MalwareBytes turned off (against my better judgment). Still the update progresses through "iPhone...cannot be updated. An unknown error occurred. (4000)."

From experience, I've learned that if I wait a few weeks until these buggy Apple updates are fixed, some day an update will appear that works. But in the meantime, the known exploitable flaws are open to hackers. I'd like to fix this problem now. I'm all ears to anyone who has a proven, tested fix. (Those who only know "Do a factory reset", please do not waste our time.)

Thanks.

DLM45