You say the mouse works, but not the pen.
Just a thought. Disconnect the mouse and see if the pen works, or if not, then reboot after disconnecting the mouse and see if the pen works.
If that is the case, then it may be possible in the control panel/settings to allow both to work when both connected.
If you haven't yet done so, it is also worth doing a Google search for both UGEE GROUPS and GIMP 2.8 GROUPS, as other users will probably have come across this problem before. Or even a more general search for UGEE GRAPHICS SOFTWARE.