asp.net webforms app with mvc, unable to find "/Account/Login"

I have an asp.net webforms application with later on added mvc component. The login view is embeded inside the homepage Default.aspx. When I type into browser and try to open a login controlled URL without loggged in, i should be redirected to the Default.aspx homepage for login, but instead I get 404 not found error looking for "/Account/Login".

The solution worked for me is rather wierd:
add the following setting in AppSettings section of your web.config