@endlessly I had the same problem as you, I fixed it this way:
1. Your calling to render the modal must be outside <body>, <nav> or any tag, for example mine:
<nav>
<%= linkto 'Sign up', signin_path, {:remote => true, 'data-toggle' => "modal", :class => 'nav-item nav-link', 'data-target' => '#modal-window'} %>
</nav>
@endlessly I had the same problem as you, I fixed it this way:
1. Your calling to render the modal must be outside <body>, <nav> or any tag, for example mine:
<nav>
<%= linkto 'Sign up', signin_path, {:remote => true, 'data-toggle' => "modal", :class => 'nav-item nav-link', 'data-target' => '#modal-window'} %>
</nav>
<div id="modal-window" class="modal" tabindex="-1" role="dialog" aria-labelledby="gridModalLabel" aria-hidden="true">