Thank you for these suggestions. I have
- Tried dozens, if not hundreds, of times.
- Logged in successfully from many other places, including Web, Outlook on my desktop (which as far as I can tell is using the exact same server settings), and Blackberry Hub on my PRIV.
- Removed all approved devices and reactivated them individually (including desktop and PRIV). They now show up again in my account.
I have also
- Completely reset my phone using the Windows Device Recovery tool.
- Turned off every security setting in Yahoo!
No dice. L950 won't connect. Everything else I own will. Something is wrong with my particular device...but there is nothing else particularly wrong with it that I can tell (excepting all the normal, innumerable, W10M annoyances).