pfSense and SharePoint Intranet

I recently implemented my first pfSense box to replace an old cheapo router. After configuring the firewall, WAN users were able to access our SharePoint site (http://dc.ourcompany.com), but LAN users were redirected to a pfSense page stating, "Potential DNS Rebind attack detected, see (wikipedia page) Try accessing the router by IP address instead of by hostname."

I tried disabling the DNS Rebind Check, which caused internal requests to the SharePoint site to hit the pfSense login page.

Eventually I stumbled upon a suggestion to "Disable NAT Reflection for 1:1 NAT," which allowed both WAN and LAN users to access the SharePoint site using http://dc.ourcompany.com.

Questions:

1. Any idea why internal requests were hitting the router/firewall instead of the webserver?

2. Is there a better way to accomplish the desired outcome?

3. What impact will my changes make to the security of our network?

Thanks in advance!