That's the funny thing. The backupmenu item was present in /etc/bootmenu.d, but not shown in the menu at boot time. I uninstalled backupmenu and the file was not deleted. So I deleted it manually and reinstalled beckupmenu a number of times, but the file was not recreated.

Yup, both present and looking OK.

Another thing. Bootmenu has a USB mass storage mode. It says at the bottom on the screen to press Ctrl to enter it. So I did and it went to the mass storage mode, as indicated by my PC popping up a file explorer window, but then the phone reboots spontaneously. Is it supposed to do that?

Perhaps I should mention that all this started happening after an unexpected reboot while I was compiling something on the device. Presumably caused by the watchdog, the CPU usage was pretty high. Is there a way to disable that darned watchdog? It's causing nothing but trouble.