I now have the webmail working, but I can's say conclusively which adjustment "fixed" it. I was continually testing it all day, but ut wasn't until step a that I had an improvement.
In reverse order, as best as I can remember now:
a:
made a new user , set the password, and was immediately able to log in. All past attempts at logging in failed for existing users.
b:
restored the file /etc/php.ini from another e-smith install, and edited it to accomodate the PHP upgrade how-to from Dan Brown.
service httpd restart
service mysqld restart
c:
reinstalled the current 4.1.2 openssh rpm from the update section.
d:
Turned webmail off, on, http, http & https
So, I can't tell you what to attribute this failure to other than a recent flurry of add-ons. I meant to log each one, and test the results before moving on. I will start doing that again, but till then, I think the system at work is stable.
I still have to fix the system at home, so I will try these steps again and post results