Well solved my issue myself with some help from this thread! Thanks for the clue, nant!
In my case the field 'lastpasswordchange' is NOT a core field; it's added when you install the Brightcode Password Changer. I installed this briefly to test it, but I have long since uninstalled this (since the 'friend' that 'shared' it with me neglected to mention it was a commercial subscription app - some friend, huh?
) but apparently it never dropped the field from the table.
So the key here is that if you are getting store errors on profile updates, pay close attention to the field named and see if you can trace it back to a 3PD component that extended the core user fields.
I tried searching the J! forums, extensions, etc. to see if I could find where the 'alertKeywords' field came from , with no luck so far. But I would look for some component that extends the User table functionality.
Hope this helps!