MARKED AS SOLVED BUT NOT REALLY - the keyboard started working again after a couple of weeks. I have no idea why, nothing has changed in the setup / connections / software or configuration. So much for Apple products that "just work".
I use a MacBook Pro 15" 2019 (running BigSur 11.6.5) supplied by my employer as well as a personal linux PC, switching between them with a KVM.
I've been using my USB keyboard (Fnatic Gear RUSH Red Silent) for nearly three years without issue on both systems, but 2 days ago the Mac has completely stopped working with the keyboard and I can't understand why:
- typing any key on the keyboard has no effect at all, including all letters, numbers, modifiers, arrows etc
- the keyboard is recognised by MacOS and shown in the Keyboard control panel (clicking on "Modifier keys" shows a dropdown selection for connected keyboards, including mine). It's definitely not a connection issue or a device issue, as it's clearly recognised as a keyboard
- if I click the "Change keyboard type" button, the standard dialog asks me to press the key to the right of the left Shift key (as I did when I first connected the keyboard years ago), but now when I press the key, nothing happens
- the keyboard works fine on other machines
- I have to enter a boot password as the disk is encrypted; at this password prompt, the USB keyboard works fine. It stops working once the OS is loaded
- I have tried direct connection (with a USB A to C adapter of course) to all 4 ports, as well as the connection I normally used which was never an issue before (through a USB dock and then the KVM which is effectively a USB hub)
- I have tried disconnecting everything else and connecting only the keyboard, and it still didn't work
- another USB keyboard connected in any way (direct through the same USB A->C adapter or via dock+KVM) works fine. It's a crappy dome keyboard and I would like to use my nice mechanical keyboard instead.
- I have reset the SMC and NVRAM
- I have culled basically any files that match ~/Library/Preferences/com.apple.keyb as well as /Library/Preferences and rebooted
- the keyboard is still not recognised, even in Safe Mode
- I normally use UK PC layout (not UK Mac). I have tried various layouts and they make no difference
- I cannot and will not update to Monterey at this stage, because a) my employer doesn't fully support it yet, and b) this setup worked fine on BigSur until 2 days ago. I don't want to upgrade with everything that entails, on the remote chance that it might magically help (there's no reason at all why it should)
- BONUS POINTS: the RUSH keyboard has a built-in USB hub. The other USB keyboard works fine through this USB hub, but keypresses on the RUSH still do nothing
- MORE BONUS POINTS: I've noticed some Fn+ combinations work and are recognised as multimedia button presses. The keyboard has an Fn key and in combination with F1->F6, these produce multimedia control keys. This works.
I suspect this may have something to do with "mapping" keys to keycodes or something like that. As detailed above, I have tried setting different keyboard layouts but it's made no difference.
I have tried contacting Apple Support who couldn't solve the issue. If anyone has any suggestions I would greatly appreciate the help.