Grub issue (Libreboot)

I have installed Libreboot on a T60.
When booting I get messages like this:

Welcome to GRUB!
error: variable 'prefix' isn't set
error: variable 'prefix' isn't set
error: variable 'prefix' isn't set
error: no device connected
error: no device connected
error: no device connected

From the grub menu 'Search ISOLINUX menu (USB) [u]' does not work (with a live usb pen drive which has been tested to be functional on a similar machine)

Searching the web for 'libreboot grub error prefix isn't set' gives nothing I can understand.
The problem may be unrelated to libreboot since I see others having faced similar issues on non-librebooted machines running other distros, e.g. arch, debian etc

I wonder what the error messages mean and how to fix it.