I experienced the same issue after I upgraded my 8.0 to the 8.1 Pro Preview. I'm using Juniper 6.5.0.
I resolved it by:
1) Delete the "phantom" Juniper Network Connect Virtual Adapter
2) Uninstall Juniper
3) Reinstall Juniper
I had considerable difficulty removing the driver. I could not uninstall it directly from Device Manager, such that I would run the uninstall but the driver remained. I went in to the registry but was unable to delete the folder due to
lack of permissions, even though I was the owner of said folder. So, I removed all the values, which was enough to make it appear uninstalled.
After the first attempt at reinstallation post-registry change above, it created a "Juniper Network Connect Virtual Adapter #4" driver entry in device manager.
I then was able to delete that directly from Device Manager then did the Juniper reinstall and it worked right away.
I should also mention that the virtual driver is no longer present in Device Manager, so my theory is that the 8.1 upgrade misinterpreted Juniper to need a device driver so created a device driver that was causing the issue. My Win 8.0 laptop also
does not have the driver so it really wasn't ever needed.
Let me know if I can provide more detail.