Hi there,
I would like to report that I have had the same issue twice in the last two days.
I cannot yet say what exactly needs to happen to reproduce it, but here is what I have observed so far:
In both cases, it happened several hours into the flight, nearing TOD. Yesterday it was a flight from LIRF to HUEN, today it was LSZH to GCTS
For contrast, last weel I did a flight from EDDH to KBOS where this did not happen - I don't think there was an update to the A310 in the meantime
In both cases where it happened, I was fidgeting around with the FMC. In both cases I think it happened right after I pressed one of the keys to the right of the display. Yesterday I don't remember what page I was on, but today I was on the FLTPLN page and wanted to set a lower altitude constraint for one of the upcoming waypoints.
I did not try to activate the secondary flight plan (in fact I haven't yet found out how to do that), but today I had copied the primary flight plan - I am not sure whether I had done that yesterday.
The effect in both ocurrences was that no cockpit dial, button or switch could be used anymore. The aircraft continued flying, although its capability to hold altitude deteriorated.
I "solved" the issue by saving the flight, going back to the menu and then loading the flight I had saved. This is not too god a workaround though - I could continue the flight afterwards, but I had to reconfigure most of the aircraft after reloading and also reprogram the FMC. In addition, although I could complete the flights in both cases I had the feeling that the aircraft was rather hard to fly manually in the final approach later, requiring strong aileron inputs to keep it on runway course. Especially the landing in HUEN was not much fun...
Anyway, I hope this account helps to narrow it down, and in any case I wanted to make it known that this seems not to be an isolated incident. I will continue observing what makes this reproducible (and it may be the secondary flight plan), but there seems to be an issue somewhere and I would venture to say it's connected to the FMC.
Nevertheless, congratulations for an otherwise pretty neat airplane - and thank you for providing it!