Forms Authentication not working ASP.NET 4.6.1 / VBNET / (VS 2015) and IIS 7.5

I have a clean asp.net web forms (4.6.1) site built using VS 2015 on IIS 7.5 Server 2008R2 with SQL 2012

I have added a couple of pages, created a new master and css - all works fine.

I want to use forms authentication - (it is an internally hosted site but we want customers to have access) but once you change the authentication mode in web.config from none to anything else, it redirects to account/login and tells me

HTTP Error 404.15 - Not Found

The request filtering module is configured to deny a request where the query string is too long.

I have added

 <appSettings>
    <add key="PreserveLoginUrl" value="true" />
    <add key="autoFormsAuthentication" value="false" />
    <add key="loginUrl" value="logon.aspx" />
    <add key="enableSimpleMembership" value="false" />
  </appSettings>

<authentication mode="Forms">
      <forms name=".ASPXFORMSDEMO" loginUrl="logon.aspx"
      protection="All" path="/" timeout="30" />
    </authentication>
    <authorization>
      <deny users ="?" />
      <allow users = "*" />
    </authorization>

Still no joy.

Anyone any suggestions as to how I can change a vanilla asp.net site to use forms authentication getting names from a SQL database? Just getting it go go to the correct form would be a start!

Nick Bridgens