Hi all,
A fix for me was to adapt the roots_gallery function (can you overwrite it?). The function can be found in themes -> roots -> inc -> cleanup.php. You have to wrap a an element (I used div) with the class ‘gallery’ on it around the ul-element of the list of thumbnails. Then you get output like this:
<div class="gallery">
<ul id="gallery-1" class="thumbnails gallery galleryid-28 gallery-columns-7 gallery-size-thumbnail">
<li class="gallery-item">
<a ...>
<img ...>
</a>
</li>
</ul>
</div>
Hope it helps!