For the described I am using an IBM model M keyboard (PS2) which
is connected with a "blue cube" OS2->USB adaptor to a USB-port
of my PC.
this all had worked before:
When grub comes up, I entered the password to access the
grub "command lines" for the kernel boot, add an "S" at
the end of that line, hit enter and "B" for boot. The system
comes up and asked for the root password or to hit CTRL-D
to boot into normal mode.
Now, I can enter the grub and add an "S" to the commandline
and boot the kernel by pressing "B".
When the kernel asks for the root password, the keyboard is
no longer recognized. No chance other than to press the