Login form producing msgmode=4 error

Hi All,

I have a customer login form in my header file which is pretty much set up like the pc manual.

            <%if session("idCustomer")="0" or session("idCustomer")="" then %>
                    <li>
                    <form name="login" method="post" action="checkout.asp?cmode=1" style="font-size:x-small;">
                    <table width="100%" border="0" cellspacing="1" cellpadding="1">
                       <tr>
                          <td><span class="style1">Username </span></td>
                       </tr>
                       <tr>
                          <td><input type="text" name="LoginEmail" style="font-size:x-small;"></td>
                       </tr>
                       <tr>
                          <td><span class="style1">Password    </span></td>
                       </tr>
                       <tr>
                          <td><input type="password" name="LoginPassword" style="font-size:x-small;"></td>
                       </tr>
                       <tr>
                          <td>
                            <input name="PassWordExists" type="hidden" value="YES" >
                            <div align="center">
                                  <a href="JavaScript:document.login.submit();"><img src="images/login.gif"></a>
                            </div>
                            <input type="hidden" name="SubmitCO.y" value="1">
                        </td>
                       </tr>
                     </table>
                    </form>     
                </li>        & nbsp;
                <%end if%>
When I try to log on this way 3 out of 4 times I am faced with a msgmode=4 error, but when I login via checkout.asp?cmode=1 everything works smoothly. What gives?

Thanks in advance,
Egemen