Time Machine: Backup Not Completed

Shortly after Time Machine starts a backup, a dialog box comes up:

Backup Not Completed

Time Machine couldn't back up to "Bk4"

If I hover the mouse over the window, a button appears: "Details"

If I click on the button, another dialog box appears:

Time Machine couldn't complete the backup to "Bk4"

An error occurred while preparing the backup.

I am running Monterey 12.4 (latest available at this time) on a Late 2015 iMac with Intel processor. I have been running Time Machine successfully for years, using a series of external disks. I recently bought and added a number of new disks, including 2 to replace my previous, failing backup disk (which I removed and discarded). Time Machine worked fine for about 2 months. Four days ago, I started getting the above messages (with the name of the backup disk changing; Bk4 is an example), and have not gotten a successful Time Machine backup since.

I've tried:

Halt syncing of Google Drive and quit.

Run Disk Utility>First aid on backup disks and internal disks, with Mac in Recovery mode.

Use just 1 backup disk instead of 2.

Remove all external disks from system except backup disk.

Free up space on internal hard drive. It had been 95% full, now just 60% full.

Erase backup disk using Disk Utility, and start Time Machine afresh.

Format backup disk as APFS rather than APFS (Case-sensitive)

Connect backup disk directly to USB port on computer rather than through a USB hub.

Try an older, smaller disk for backup.

Time Machine invariably failed, with the error messages above. The disks check out fine in Disk Utility and for other uses.

I'm now using 3rd-party backup software. It required some tweaking (including APFS case-insensitive formatting, connection of drive directly to USB port on computer) but otherwise is working fine.