forms authentication redirect to logon.aspx page - receives the error access denied

Starting page from VS2013 = manageusers.aspx

URL in browser redirects me to the logon page as it should but instead of displaying the logon page
I receive an Access Denied error.  
Redirect URL:  http://localhost:2226/logon?ReturnUrl=%2fmanageusers

Browser Error:

Server Error in '/' Application.

Access is denied.

Description: An error occurred while accessing the resources required to serve this request. The server may not be configured for access to the requested URL.

Error message 401.2.: Unauthorized: Logon failed due to server configuration. 

Verify that you have permission to view this directory or page based on the credentials you supplied and the authentication methods enabled on the Web server. 

Contact the Web server's administrator for additional assistance.

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.6.81.0

Web.config contents extract: 

<authentication mode="Forms"><forms name=".ASPXCOOKIEDEMO" loginUrl="logon.aspx" defaultUrl="mainmenu.aspx"
             protection="All" timeout="30" path="/" requireSSL="false"
             slidingExpiration="true" enableCrossAppRedirects="false"
             cookieless="UseDeviceProfile" domain=""><!-- protection="[All|None|Encryption|Validation]" --><!-- cookieless="[UseUri | UseCookies | AutoDetect | UseDeviceProfile]" --></forms></authentication><authorization><deny users ="?" /><allow users = "*" /></authorization>

To get around the error but not fix it:

If I remove this line:       <deny users ="?" />

then the redirect to logon.aspx does not happen

but I can get into the application pages?

Any help would be great.

Thanks

Graham