Bug#984520: 'error: symbol "grub_register_command_lockdown" not found' and then lightdm fails to start

[Colin Watson]

On Thu, Mar 04, 2021 at 05:18:18PM +0100, Marco Kühnel wrote:
> after the recent update to grub2/202+dfsg1-20+deb10u4, during boot the mentioned error is thrown. After that, boot continues but lightdm cannot be started anymore.
> This only happens on my laptop containing two bootable hard disks. The separate boot partitions share /boot/efi and from the second hard disk its Ubuntu 
> installation still boots flawlessly. For the Debian installation, /boot and /boot/efi are both on /dev/sda, but by default the laptop boots from /dev/sdb.

What do you mean by "the laptop boots from /dev/sdb"?  If your EFI
System Partition is on /dev/sda1, then by definition that's where the
boot loader lives, unless you're doing something else odd.  Do you just
mean that your root file system is on /dev/sdb?

> Tried: grub-install /dev/sda

In UEFI mode, grub-install ignores the device name you give it, and
always installs to the EFI System Partition on /boot/efi.

Could you please post the full output of "grub-install --debug"?

-- 
Colin Watson (he/him)                              [cjwatson at debian.org]