Hallo.. this is easy to resolve.
This with the external IP do-sent work on all Routers.
just some router are able to reroute a request from internal network to the external IP address back.
To fix up.
first check your external URL Host entry for your external IP:
add a Host entry on your external DNS Server. eg: erp.mycompany.com --> externeal-IP
Put this external Host adress inside your ERP under parameter/systzem parameter/web_base_URL
after this your ERP scholt by work from external.
for internal.
Create on your Domain Server AD-DNS new Master zone with the same Dmain name as your external Domain name.
ther you have entered in (web_base_url).
Create a reverse zone on your network Subnet range.
eg: 192.168.10.x
Now add host name to your DNS master zone erp with the internal IP address.
new boot DNS converting the name erp.mycompay.com on internal network to internal ip ore from external to the external IP address.
Openerp tells every time the base URL to the Browser for all links.
Yust make sure in your DHCP server is the primary DNS server the internal DNS-Server as primary set.
The internal Clients most resolve the IP address from Internal DNS Server.
When you try to access the OpenERP server from within the LAN, are you using the Public IP address or the internal IP address of the server. It is good that you checked the firewall but there must be some other networking issue.
For remote access you forwarded port 8069 through your router to the static IP Address of the Win2k8 server. When you try to access from the LAN are you using the same IP Address as your router has port forwarded? Using the same LAN PC that fails can you access OpenERP through your public IP Address or does your security architecture disable round-tripping?