your forums app doesn't know what that control is or where to load it from, it is probably throwing an error but the error is handled. I'm not sure if it could even load the control outside its own application boundary.
You could try to declare it at the top of the master page like this:
<%@ Register TagPrefix="portal" TagName="SiteMenu" Src="~/Controls/SiteMenu.ascx" %>
but I'm not sure it can load it.
The SiteMapDataSource would also have to be present.
Hope that helps,
Joe