Below are the files as far as I know that play a role in this. I have omitted comments contained in the files and have only shown what is relevant. The custom.cfg file has been newly added due to the new OpenBSD menu entry, and the rest has been returned to its original state as shown.

/etc/default/grub:
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=MENU
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

/etc/default/grub.d/50_linuxmint.cfg:
#! /bin/sh
set -e
GRUB_DISTRIBUTOR="ubuntu"
GRUB_DISABLE_OS_PROBER=false

/boot/grub/custon.cfg:
menuentry "OpenBSD" {
set root(hd1,4)
chainloader +1
}

I don't know how to address the OpenBSD kernel directly instead of via chainloading.

It seems others do: Google.

When 'c' is pressed I don't get a command line. If I press 'c' in the grub menu, the menu bar jumps to OpenBSD and stays there, neither the arrow keys, some TABs, or any other keys can move the menu bar. The latter also applies if I don't press 'c'. Both OS's can be booted, only the arrow keys do not work. As the article indicated, I created a /grub directory in OpenBSD and placed the suggested file in it. After a reboot, nothing has changed, so the error remains as it is.

Are we dealing with an MBR installation here, or UEFI? If UEFI, what does tree -ha /boot/efi report? (adjust location as required if rescue booted.)

Is Mint's (Ubuntu's) current Grub 2.06 the only Grub that has ever written to disk? If not, what did, and how long ago? Maybe it's time to reinstall Grub, not just update grub.cfg, custom.cfg and/or /etc/grub.conf.d/. ???

This is a MBR install, not an UEFI.

Its grub v 2.04. It is a fresh install, Linux mint 20.2, then upgrated to 21.2. How do I install grub v 2.06?

Did you try plugging the keyboard into various USB ports? Check they were all unobstructed? Using a USB3 keyboard? USB3 port? Enable Legacy USB support or USB keyboard support in the BIOS? Do the Page UP/Down keys work? Do the arrow keys work when you access the BIOS?

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The arrow keys work in the BIOS, as well as in programs such as nano and less in OpenBSD and Linux. Other keys also work normally. USB is V2.0, both keyboard and connections. Therefore, it cannot be due to the USB connection or the keyboard.

The Mint 21.3 Virginia I currently have booted has Grub v2.06-2ubuntu14.4. You may not need to do anything more than get your Mint fully up-to-date.

I updated V21.2 to 21.3, but still have grub V2.04. I don't know how that is possible, but the question remains: How can I upgrade/reinstall grub?

what will apt policy grub2-common say?

grub2-common:
Installed: 2.06-2ubuntu7.2
Candidate: 2.06-2ubuntu7.2
Version table:
*** 2.06-2ubuntu7.2 500
500 http://archive.ubuntu.com/ubuntu jammy-updates/main amd64 Packages
100 /var/lib/dpkg/status
2.06-2ubuntu7 500
500 http://archive.ubuntu.com/ubuntu jammy/main amd64 Packages

But above the menu it says: grub v2.04.