If there is a problem with SSO, the first thing to do is check the webmail log file msg-svr-base/log/http for errors. Increasing the logging level may be helpful (configutil -o logfile.http.loglevel -v debug). If this does not help, then check the amsdk messages in msg-svr-base/log/http_sso, then increase the amsdk logging level (configutil -o local.webmail.sso.amloglevel -v 5). Note that new logging levels only take effect after server restart.
If SSO is still having problems, make sure you are using fully qualified host names of both Access Manager and Messaging Server during log in. Cookies are only shared between servers of the same domain, and browsers do not know what the domain is for local server names, so one must use the fully qualified names in the browser for SSO to work.