Media Keys Not Working for a Specific User Account

I have macOS setup with several user accounts for different purposes, and in all but one of them the media keys (play/pause, previous, next, and volume up and down) are all working correctly.

However, in one specific account the keys have mysteriously stopped doing anything and I can't for the life of me figure out why. There is no obvious cause.

I'm using an Apple Magic Keyboard With Numeric Pad, with a 2018 Mac Mini running macOS 10.15.3, but the issue began under 10.15.2 (I had hoped the update/restart would fix it).

Other things I've already tried:

1. I've checked the behaviour under System Preferences -> Keyboard (it doesn't seem to be a function key issue, as the media keys don't work either pressing the Fn key or not).
2. Reset NVRAM
3. Reset SMC

The fact that this is only affecting one specific account is the part that has me confused; of all my accounts it's the one I would least expect to have a problem, but the one in which it is most annoying, as it's the one I use for watching TV shows/movies (i.e- minimal running programs, no distractions etc.).

Also it's strange that the issue is affecting both the media keys and volume keys; I've noticed all kinds of annoyances with macOS' bonkers media key behaviour with apps fighting for focus so in any given moment I have no idea what play/pause will actually do, but I've never had any issue with the volume control keys, which also do not work for this user account.

Ive also checked my keyboard shortcuts; but I only have shortcuts for other keys (F5 and F6, and F13, F14 and F15, i.e- all the "blank" function keys on my keyboard that do not have special behaviours).

Now, if there's no other way to resolve this issue, I'll of course have to remove and recreate the user account; not the end of the world, but I'd very much like to avoid having to do this if I can.

Does anyone know of any likely reasons why a single user account would have non-functioning media and volume keys, when all others are functioning normally, and how I would go about fixing it?

So I typed this whole thing out and the culprit presented itself right after I posted it!

I have a trackball by Elecom, love it, but apparently there was a problem with their Elecom Mouse Assistant software; an error message popped up, so I force quit it, trashed its settings and relaunched it, and now my media keys are fine again.

It must have something to do with the ability to remap the mouse buttons to various functions (volume and media functions being some of them).

For anyone that experiences the same problem, and has Elecom Mouse Assistant installed, you may wish to try:

1. Relaunch the app:
   1. Open mouse assistant (from System Preferences)
   2. Make a minor change (e.g- assign a mouse wheel click function), and click Apply.
   3. Check your media keys, this may have resolved your issue.
   4. If necessary, re-open mouse assistant and change your settings back to the way you wanted them.
2. If that doesn't work, you may try the following:
   1. Locate your settings file and delete it. This appears to be a hidden XML file stored under your user directory, so it will most easily be found using Terminal:
   2. Open Terminal.app
   3. Enter the following command and hit enter: ls -al
   4. Look for a strangely named file with the .xml extension. I'm not sure of the naming convention, it may be model specific, but mine is: .VID056e_PID00fc.xml
   5. Remove the file using the rm command, substituting in the correct name for the file, like so: rm ~/.VID056e_PID00fc.xml
   6. Logout of the user account, and then log back in again

Using either option your normal use of your media and volume keys should be restored without having to uninstall mouse assistant.

I won't mark this as the right answer, but may leave the question in place in case anyone else has the same issue, or has other causes they would like to document for others.