Why is my system not installing newest kernel (linux-image-5.4.0-84-generic)?

I'm using Ubuntu 20.04.3 LTS x86_64 and I've been trying to install the newest version of the kernel using:

sudo apt update && sudo apt upgrade

But by the end of the run it always shows the following message:

Adding boot menu entry for UEFI Firmware Settings
/etc/grub.d/35_fwupd: line 5: /usr/share/grub: Is a directory
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 126
dpkg: error processing package linux-image-5.4.0-84-generic (--configure):
 installed linux-image-5.4.0-84-generic package post-installation script subprocess returned error exit st
atus 1
Errors were encountered while processing:
 linux-image-5.4.0-84-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)

The output of uname -r is:

5.4.0-81-generic.

That means that the kernel isn't correctly installed.

Does anybody have an idea of what's going on?

Edit:

By request I'm posting the content of my /etc/grub.d/35_fwupd file:

#! /bin/bash
# SPDX-License-Identifier: LGPL-2.1+
set -e

${pkgdatadir:?}
# shellcheck source=/dev/null
. "$pkgdatadir/grub-mkconfig_lib"

if [ -f /var/lib/fwupd/uefi_capsule.conf ] &&
   ls /sys/firmware/efi/efivars/fwupd-*-0abba7dc-e516-4167-bbf5-4d9d1c739416 1>/dev/null 2>&1; then
      source /var/lib/fwupd/uefi_capsule.conf
      if [ "${EFI_PATH}" != "" ] && [ "${ESP}" != "" ]; then
      echo "Adding Linux Firmware Updater entry" >&2
cat << EOF
menuentry 'Linux Firmware Updater' \$menuentry_id_option 'fwupd' {
EOF
      ${grub_probe:?}
      prepare_grub_to_access_device '`${grub_probe} --target=device \${ESP}` | sed -e "s/^/\t/"'
cat << EOF
    chainloader ${EFI_PATH}
}
EOF
      fi
fi

According to the user Ahmed & ubfan1, the file /etc/grub.d/35_fwupd isn't part of the Ubuntu apt-file search, and could be a leftover from an upgrade I had recently from Ubuntu 18.04 LTS.

After deleting that file and trying to upgrade again the error message was gone and the output of uname -r now displays:

5.4.0-84-generic

After that I went through a couple of reboots to see if there would be any problems, but nothing so far.