Grub Password not working

Hi,

on my x86 openwrt router I use the 21.02.3 combined squashfs image.
I'm trying to set a grub password by adding the following lines to grub.cfg:

set superusers="root"
password root root

Also I add the flag --unrestricted to default menuentry. My whole /boot/grub/grub.cfg looks like this:

serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1 --rtscts=off
terminal_input console serial; terminal_output console serial

set superusers="root"
password root root

set default="0"
set timeout="5"
search -l kernel -s root

menuentry "OpenWrt" --unrestricted {
        linux /boot/vmlinuz root=PARTUUID=5a758c35-a6c1-e37d-b819-777f56e24302 rootwait   console=tty0 console=ttyS0,115200n8 noinitrd
}
menuentry "OpenWrt (failsafe)" {
        linux /boot/vmlinuz failsafe=true root=PARTUUID=5a758c35-a6c1-e37d-b819-777f56e24302 rootwait   console=tty0 console=ttyS0,115200n8 noinitrd
}

This should add a user root and its password root.
Now grub asks me for username and password when editing an entry.
But I'm not able to login.

Do you have any ideas?