The server method '?' failed

Hi,
Please help before I have no hair left! I'm trying to call a serverside webmethod from javascript (never done that before so this could be obvious) using PageMethods and I'm getting the error "The server method 'callserver' failed. I can run PageMethods.callserver('doris', OnSuccess, OnFailure); in the firebug console and it works fine. Here's me test code:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Single.aspx.cs" Inherits="Single" %>

<!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">
    <title></title>

    <script type="text/javascript">
        func ...