Not quite sure who’s replying to what here, but to clarify:
I’ve tried different USB ports: some of them are straight into the motherboard; some are on a PCIe card; some are on a USB hub. All behave the same; I’ve also re-installed drivers for Midisport.
Works flawlessly with Cubase; used to work with Dorico 1.
I suspect it’s something to do with the Halion Sonic SE3 plug in: when that starts to load as part of the Dorico launch process, the MIDI activity lights on the Midisport go dead and playing keys on my keyboard no longer lights that up. Looks like its switched the Midisport “off” - the power light stays on but all activity from the Midi indicator lights stops. (I’ve tried using port B as well before anyone asks!)
Then when I start to use Cubase, it all starts up again.
If I could launch Dorico without launching Halion Sonic SE3 I would try that - but I can’t see a way of doing that.