After Creating new Site Collection root(SPWEB) with Host Header every request made from server (internal) is denied (Popup login Window continually). If requests come from FE (same USER), access is granted.
You might be victim of LoopBackCheck.
- Bypassing your proxy server for local addresses (Source)
- Adding a team Web site to the list of trusted intranet sites (Source)
- SharePoint disable loopback check (DisableLoopbackCheck dword in registry)(Source)