Hello Rob,
I've been meaning to try DOSBox-X as I noticed it has more features. Anyway, I resolved the unhandled exception in this app. What I did was I found an updated MouseKeyboardActivityMonitor DLL file by downloading the *.nupkg file from here (click Download Package on the right side):
https://www.nuget.org/packages/MouseKeyboardActivityMonitor
The *.nupkg file opens up with 7-zip. Go to the \lib\net20\ folder, copy the *.DLL file and overwrite the existing one.
The app starts with no error and allows you to program keys to the up and down for the scroll wheel.
Now I just have to find a game that it works in because I was expecting the scroll wheel to replace the function of the trackball when moving up and down or forward and back, but didn't work out that way.
That's how I would like it to work.
Steve