boot error, cannot upgrade kernel

[Gary Aitken]

Hi all,

On a new Framework laptop, I installed ubuntu 21.10.
All seemed (mostly) fine; in particular, I could boot...

I then attempted to install the rEFInd multi-boot manager, but when I
rebooted I got an error; I *think* the error is the same error described
below, but I'm not certain.  I then uninstalled rEFInd and things were
again ok.

Subsequently, I have tried to upgrade the kernel; I've tried both 5.14.21
and 5.15.6.  They install ok, but when rebooting, I get:
   Error: can't find command  'hwmatch'
   Error: file /boot/vmlinuz-5.15.6-091906-generic has invalid signature
If I then boot into 5.13.0-22 (recovery mode) (the original kernel) using
the "Advanced" boot option, and select "fsck", I get:
   /lib/recovery-mode/recovery-menu
     line 80: /etc/default/rcS:  No such file or directory
   fsck from util-linux 2.36.1
   /dev/nvme0n1-2 is mounted
   e2fsck: cannot continue, aborting
As complained about above, there is no /etc/default/rcS, but there is one
in *some* of the snaps:
   /snap/core/11993/etc/default/rcS   (/snap/core/current points here)
   /snap/core/11743/etc/default/rcS
   /snap/core18 has none
   /snap/core20 has none

There is a file /usr/lib/grub/i386-pc/hwmatch.mod

Can anyone tell me what's going on, and some clues for fixing it so I can
upgrade the kernel?  (I've validated the checksums on the kernel files).
Right now I have to uninstall them using sudo apt purge linux-*5.15.6* to
get back to a normal boot.

Thanks,

Gary