I'm not sure if this is a Logitech issue, a Dopus issue, a Windows 10 issue, or a something else issue, but I'm posting this in case it helps any Logitech mouse users.
Last couple of times after installing newer versions of Dopus, the middle click on my M500 mouse has stopped working. Reinstalling the drivers didn't fix it.
But I found what did. Open the Setpoint app in Mouse and Keyboard Settings - the one where you assign button actions. You don't need to do anything in there; just open it and close it. And immediately the middle button starts working again.
Don't throw out your mouse without trying this fix