Ups; sorry... i didn't read carefully your first question and jumped a bit too quickly on the // problem.
In fact, your problem may just come from an error in your nagios config files. You should check them using the:
nagios-dir/bin/nagios -v /nagios-dir/etc/nagios.cfg
It will tell you what the problem is, if there is one.
If it doesn't solve your problem, you may refer to the FAQ on the officiel nagios website:
nagios.org/faqs/viewfaq.php? ... desc=false
And if it still doesn't work, post again here
I tried to call the tactical overview with /nagios/cgi-bin//tac.cgi on my computer and it worked ...
so the // may not be the problem