Once you have done the nagios client installation and configuraiton in your system then, you may getting an error HTTP-WARNING : 403 Forbidden like that same as,

Check your nrpe configure file : check_http

#  /usr/local/nagios/libexec/check_http -I host.hostname.com -w 5 -c 10 -p 80

HTTP WARNING: HTTP/1.1 403 Forbidden - 659 bytes in 0.032 second response time |time=0.032355s;5.000000;10.000000;0.000000 size=659B;;;0

The Nagios checking into your webpage (Root Directory), you need to add index.html or index.php file.

Note :
Check your websites by the ip address or domain name its shows 403 forbidden error.

# /usr/local/nagios/libexec/check_http -H host.hostname.com -w 5 -c 10 -p 80

HTTP OK: HTTP/1.1 200 OK - 394 bytes in 0.007 second response time |time=0.007302s;5.000000;10.000000;0.000000 size=394B;;;0

Do you need more information about HTTP or HTTPS  monitoring