This error occurs on 1.3.9h and f
sites have SSL Configured.
ultimate seo urls 2-110 installed

to reproduce the error:
open browser, type in site with www , then click log in then in menu bar or main page heading. if you can log in successfully close browser then,
open browser, type in site with out www , then click log in , when you enter your username and password , the page resets to home page with out logging in.
if you then click the home button then log in, it will work. then close browser go back to site with www and log in fails till you click home button again.

This also causes customers to loose carts if they shop first then log in.

As far as I know the problems is with session ids
adding a 301 redirect to force all www or no www might fix the problem
but as the site has many customers doing this now may cause for more people to experience the issue. ( have not tested yet )

there seems to be many people experiencing the same or simular problem and many threads but I have been unable to find a solution anywhere, anyone know how to fix or does not have this problem?