When users try to login from post.asp using the login form under the top right navigation, it simply redirects to default.asp without completeing the login process.
Looking at the source code the value for action attribute of login form shows the following
<form action="/forum/post.asp" method="post" id="form1" name="form1">
This happens because the Querystring is missing from the url in the action attribute of form.
Following is part of the statement in inc_header.asp (Line 341), which results in the output above:
Response.Write " <form action=""" & Request.ServerVariables("URL") & """ ...
It could be resolved by adding the querystring to the url in the action attribute of form.