Thanks to both of you for the useful feedback. This was one of those where… things went really badly with some coursework, under time pressure, and I logged into my email late in the afternoon to download some student files and found just one more problem to deal with. It really helps me a lot to have a pointer to the answer, so thanks to you both, it’s much appreciated.
FWIW I don’t think 2FA is the problem. I’m skeptical of M$ / Github’s claim of abnormally high SMS failure rates to mainland China but… what can one do? Other than minimize contact with that nonsense. (I won’t take a position on moving the SC repository, but once I have a spare hour or two, I expect I’ll move my quarks though.)
hjh