From this forum post:

For the "Invalid Host Header" issue -- this is caused by new security measures added in this release to prevent host header injection attacks. Values that are considered "valid" for your host value are the IP address of the server as you noted, as well as "localhost" and the value you input for your "External URL" (located on the General tab of System Settings).

If you need to in order to get to the settings page, try going in via the host name instead of the ip address.

I tried nagiosxi at http://<external ip>/nagiosxi but still shows the same error

Please let me know @jmichaelson what to do ,as I  tried on acessing in external IP as well but nagiosxi is not acessible ,so please provide me step by step solution for invalid host header 

Did you try going in via a host name instead of an ip address?