“add_control_to_stack was called incorrectly”

I have spent hours trying to get Elementor working on WordPress, but with very limited success. I now get the following warning, which I do not seem to be able to avoid:
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Notice: Elementor\Controls_Manager::add_control_to_stack was called incorrectly. Cannot redeclare control with same name. – view Please see Debugging in WordPress for more information.

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I have reinstalled Elementor three times (twice by uploading the files, once installing the plug-in from the online selection)… perhaps this is what is causing the problem?

I feel like throwing in the towel and defecting to SquareSpace… can anyone help or should I admit defeat and find a solution I can master with the limited skills I have?

The page I need help with: [log in to see the link]