This is due to EventSentry not being able to execute the nslookup.exe process on the server which is needed to resolve host names to IP addresses.
You can solve this problem in two ways:
Disable the IP Address lookup feature. Navigate to MAINTENANCE - PROFILE EDITOR and select your current profile. Then, click "Edit Selected Profile". Locate the Resolve IP addresses option and set it to NO. Then click apply and all pages should load correctly.
Make sure that the user who is logged in has permissions on the server to launch the nslookup.exe process. Administrative users usually have this permission.