BIOS menu freezes at password prompt
I've searched around quite a bit for this but cannot find a solution.
When I first got my board I installed Xubuntu 18.0.4 onto the eMMC. I'd now like to boot from a Ubilinux 4 USB. At the "Welcome" screen I press F7, and am presented with "Enter Password." Pressing Enter has no effect, and attempting to enter a password ("upassw0rd") has no effect -- there is no indication that input is being received, the screen appears frozen.
What am I missing?