By editing main.inc.php
setting debug_level = 8 and log_session = true. I got a /var/log/roundcube/session file that says:
[27-Dec-2012 17:46:08 -0500]: Session authentication failed for 5uhp5ktuaec7bhoj8qgl4slli2; invalid auth cookie sent
I see there is a message about this at:
http://www.roundcubeforum.net/index.php?topic=10654.0
"turned off $rcmail_config['force_https'] and $rcmail_config['use_https'] (set both to False)"
Is this the proper action to take for this error? I already have force_https = false.