My entry to this little contest of telepaths: You are using a mouse control app (like Logitech Options), the app maps the middle mouse button to something other than the wheel, the app crashed reverting the mapping. Updating/restarting/reinstalling the app and restoring the mapping would help here.
If this is not the case, you should check if the middle mouse button still works as intended outside of NX. For example, middle-clicking an application in the taskbar will attempt to open a new instance of that application.