Pro Version Lightbox Image Thumbnails not working

I purchased TP Woocommerce Product Gallery PRO and integrated it in an existing WooCommerce site. The gallery functions are working perfectly and all images are loaded, but in the Lightbox mode all thumbnail images are missing, regardless which product I am viewing. Instead, for every thumbnail the base64 placeholder-code is shown as source, so it looks like this in the frontend:

<div class="lg-thumb lg-group" style="width: 735px; position: relative; transform: translate3d(0px, 0px, 0px);"><div data-vimeo-id="" class="lg-thumb-item active" style="width:100px; height: 100%; margin-right: 5px"><img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAiYAAAImAQAAAABPtXzrAAAAAnRSTlMAAHaTzTgAAAA8SURBVHja7cExAQAAAMKg9U9tB2+gAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHgNlmQAAdYpkfEAAAAASUVORK5CYII="></div><div data-vimeo-id="" class="lg-thumb-item" style="width:100px; height: 100%; margin-right: 5px"><img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAiYAAAImAQAAAABPtXzrAAAAAnRSTlMAAHaTzTgAAAA8SURBVHja7cExAQAAAMKg9U9tB2+gAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHgNlmQAAdYpkfEAAAAASUVORK5CYII="></div><div data-vimeo-id="" class="lg-thumb-item" style="width:100px; height: 100%; margin-right: 5px"><img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAiYAAAImAQAAAABPtXzrAAAAAnRSTlMAAHaTzTgAAAA8SURBVHja7cExAQAAAMKg9U9tB2+gAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHgNlmQAAdYpkfEAAAAASUVORK5CYII="></div></div>

I deactivated all caching plugins and the CDN but this did not help. Any suggestions on how to fix this?