I had the same error. Turning out to be a stupid mistake on my part, I had the same class on my opener and my inline div.
<a href="#popup" class="dialog">Open</a> <div id="popup" class="dialog mfp-hide"></div>
Of course, they had to be different classes:
<a href="#popup" class="dialog">Open</a> <div id="popup" class="dialog-box mfp-hide"></div>
Christian werther
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