UEFI Boot Menu

I went into my UEFI boot menu yesterday to try to boot to a USB drive and noticed the kanji-looking characters in the screen shot provided. I couldn't use the up/down arrow keys to select a different boot device. The tab key would take me to the "Enter Setup" and "Boot Default" buttons and both of those did what they were supposed to do.

Setup:
This is an Asrock X99X Killer running firmware ME FW 9.1.37.1002 and the most current UEFI/BIOS version P.340.

What I've tried so far:
1) Mouse-clicking the grey "X" above the Windows Boot Manager item, but it did nothing.
2) Re-flashing BIOS/UEFI same version.
3) Removed the only hardware item that had changed, a Corsair Strafe USB keyboard and went back to the previous one to see if the keyboard might have been the issue, but no such luck.
4) I thought there might be some drive corruption so I ran chkdisk /f on all the drives. Chkdisk didn't find anything and there was no change in the boot menu behavior.
5) I tried disabling some USB settings, but none seemed to have any affect on the boot menu.

The boot menu was working fine last week when I used it. I am able to change the boot order in full set-up mode in the UEFI, but that's a little more involved than selecting from the boot menu.

Does anyone have any ideas what else I might be able to try to get the boot menu working again?

Thanks in advance!

What's on the USB if secure boot is set and it's not in the database it won't let you boot from it

The USB had the install files for Debian 9. I tried it without the USB drive inserted and the boot menu did the same thing, so I don't believe it was the USB device.

The issue was I couldn't select ANY of the entries, not just that one. I should've been able to use the up/down arrows to select a drive, but the only key that was working was the tab key and it only selected the setup and boot default buttons at the bottom. When I pressed <ESC>, I'd get another dialog, but it was completely blank and after a couple of seconds, the machine just rebooted.

Could it be that one of the MBRs of one of the drives needs to be repaired? I've searched Google and haven't been able to find anything related (or what seems to be related) to this issue.

Thanks for the responses and suggestions. I've not yet tried setting everything back to defaults, although re-flashing pretty much did that for me. I have accessed the boot menu without USB drives attached and had the same result. I guess the next step would be to disconnect each of the internally connected SATA drives one at a time to see if a specific drive is causing the issue. It'd be helpful to know if the placement of those kanji looking characters is some kind of indicator that it is either the drive above or the drive below it.

I'm unsure of how that boot menu is populated. Does it read the boot sectors of all drives attached to the system and present only those it deems as bootable? Which could indicate at least one of the boot sectors is bad/corrupted. Does it just present all attached drives and leave it up to the user to decide which are bootable?

So, I just tried:
1) Disabled both SATA controllers so the BIOS couldn't see ANY of the drives. Same issue except all that shows is the kanji characters. Granted, this isn't unplugging the drives, but it should be effectively the same since the BIOS can't see them, right?
2) Removed the Corsair keyboard, and used a different, but still USB keyboard. I have a bluetooth keyboard I can try, but I don't think BT would be available at that point. I haven't had a PS/2 keyboard for years. Same issue, even with the SATA controllers disabled.
3) Flashed the BIOS again from a freshly formatted thumb drive and newly downloaded BIOS file, this time to a previous version and validated it rebooted into that version (P.310 in this case). No change.

The only other thing I can think of is clearing the CMOS and/or contact the motherboard vendor to see if they can suggest anything. Other than the boot menu issue, the board seems fine and appears to be functioning normally.

Pull the cmos battery.