Yes, It worked only when I use the first line of .htaccess file as
But I need to add that line whenever I create the new account. It is not possible for me all the time to check and solve it.
If that is the case, it could be because you do not have an index document for that URL. If it could list files / directories in that URL when using Options +Indexes, then you probably should try putting an index file in that folder and see what happens.
Or last try using the following step in your WHM panel go to the check Directory '/' Options at Main >> Service Configuration >> Apache Configuration >> Global Configuration. Indexes and all options are checked.