ok, that worked!
25.0.114 (64-bit) accepted the master license.
thanks...
p.s.
just after entering the license, MC seemed to lock up Ubuntu for several minutes.
i have not had UI (Plasmashell) issues ever, so i'd have to point to MC as the cause.
i had to reboot, then MC would not show itself.
i killed it and restarted it and now, it seems to be running ok.