wiggy, et al, the points are all very interesting, but; “what caused the ‘No Land 3’ alert”.
Perhaps type rated specialists.......
Err, well I may not be a specialist but as I hinted at I'm type rated on the 777 and have done a few uneventful autolands. In my time of both the 777 and the 744 which had (superficially at least ) similar architecture "No land 3's" were very very uncommon. If pushed to quantify it I reckon I've seen the message on approach no more than two or three times in over twenty years and never seen it during an auto land.
I don't see the point in me speculating in what can caused this particular "No land 3", there are a handful of failures I know of which will produce a "No land 3" but you'd need an engineer to come up with a full list...if there even is one. The AF engineers will have known the real story once they interrogated the Central Maintenance Computer post flight....maybe one of them will post here....