Hi Marc, this is usually caused by an incompatible jQuery library version. It might be your theme or another plugin that replaces the normal one that comes with WordPress on your site. Can you share a link so I can take a closer look?
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Hi Marc, sorry to say that your theme is indeed including its own jQuery version and it is incompatible with Easy FancyBox in its current state. There will be an update that includes the new fancyox3 script which is compatible with your theme but it’s not ready for release just yet.
When is your site scheduled for production? I might be able to send you a pre-release soon…