URL with "en" for english language does not work
I found the solution. Default language "English" requires the code to be enabled like the way when we add/change the code for new languages. 


Go to configuration click languages.
Click "edit" for default language "English".
Now put code language code "en" in "path prefix language code" text field." 
Save changes and access Url with/without "en" code. It will work in both cases for default language. 


Thanks.