Today I ran into a situation where I opened two modals at the same time. I was able to successfully close the top modal, however, when I went back to focus on the previous modal, the scroll would not work and instead it would scroll the background page, not the modal. After some investigation, I found a way to use javascript, css by using jquery. this is the jquery I used after the first modal was closed:

    $('#TopModal').modal('hide');
    $('#ModalName').css('overflow-y','auto');

It worked perfect. This is the CSS you can use.

.modal {
  overflow-y:auto;
}

Hope that works,

Credit to: https://stackoverflow.com/questions/28077066/bootstrap-modal-issue-scrolling-gets-disabled