The reason is that the response to the request for the URL contains non standard characters.
Example :
<FRAME SRC="../General-Info/Others/Main.asp" NAME="MainFrame" MARGINWIDTH = "0" MARGINHEIGHT = "0" FRAMEBORDER="NO" BORDER="0" BORDERCOLOR="#00FFFF">
</FRAMESET>
You can see, in the above example, characters such as
/ not following any standard. It should be
/ (numerical equivalent of /).