error: "A form tag with runat=server must exist" but there is a form tag!

Hi,

i get sometimes an very unexpected error:
"A form tag with runat=server must exist on the Page to use SetFocus() or
the Focus property"
at line: If j = 1 Then txta(i).Focus()

There is a form tag as you can see in the aspx file. Is there any other
reason for this error?
Thanks for help
Tartuffe

The aspx code is this:

<%@ Page Language="VB" MaintainScrollPositionOnPostback="true"
AutoEventWireup="false" CodeFile="~/myfile.aspx.vb" Inherits="myfile" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server"><t ...