There was missing "Read More" button because of wrong setting $button->modal = false in the plugin main file readmore.php. It should be set to $button->modal = true.
readmore button is back.. but... due to class modal-button it acts not as we wished it would be. The lightbox effect is not what we want when we press readmore button. A new pull request is created. -> #1213
tested the patch and the readmore button is back again.