There is a bug in SB.
robots.txt is often requested by search engine's spy bots. But if you try to request http://<sb-host>/robots.txt you'll be redirected to login screen and new message "File "SB/Views/robots.txt.php"" will appeared in "System log".
You can workaround this problem by creating the file /usr/local/sitebuilder3/htdocs/robots.txt
P.S. I'll report this bug to developer's bugzilla tomorrow.