Using my XP50 with Cubase 4.1 and suddenly
getting "MIDI buffer full" message. There are
absolutely no MIDI tracks involved in what I'm doing, just a few tracks of audio ONLY, so
MIDI overload seems unlikely??? All has been
working well for 10 days or so (even with this
current song) so why the sudden buffer problem?
Any clues out there?

Make sure local is turned off gtom the system menu. If that doesnt fix it, it could be your midi adapter.