You are accessing the Login.aspx
like /Account/Login/Default.aspx
which doesn't exist.
Shouldn't that be ~/Account/Login.aspx
?
<authentication mode="Forms">
<forms loginUrl="~/Account/Login.aspx" timeout="2880" />
</authentication>
How to debug?
- Make sure you can access Login.aspx properly.
- Then paste that URL inside form tag in web.config like above.