A web server may return an HTTP 403 Forbidden status in response to a request from a client for a web page or it may indicate that the server can be reached and process the request but refuses to take any further action. HTTP status code 403 responses are the result of the web server being configured to deny access to the requested resource by the client.
A common request that may result in a 403 Forbidden response is a HTTP GET request for a web page performed by a web browser to retrieve the page for display to a user in a browser window. The web server may return a 403 Forbidden status for other types of requests as well.
Do you have an anti-virus or firewall, if you have set the rules on strict it might deny access to the forum.