How to fix a user account screwed up by El Capitan?
Here's one way to fix (but I hope there's a better way):


In admin account, 

sudo (cd /Users; tar cf  )
In Users pref pane, delete the account and the directory.  (Trying to keep the directory got an error and did not delete account)
Re-create the account.  Verify in Terminal that the numeric ID is the same.
Restore the directory contents from the tarball.
If necessary, use chown/chgrp/chmod to fix metadata.


I also hope (too late for me) that someone can explain how it could have been prevented.