The problem was caused by multiple loading of the jQuery library. This is either hard coded into the header, or other plugins might queue up different versions of jQuery. Please not that this might cause numerous problems on your site – as loading jQuery multiple times resets the previous state, thus any jQuery plugins loaded going to be removed.
To help you, I have highlighted the multiple instances, if you decide to investigate the problem.
Instance 1: https://i.imgur.com/OiTsxIU.png
Instance 2: https://i.imgur.com/uCUiC1F.png
Instance 3: https://i.imgur.com/Gw61G80.png
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