I tried with Chrome Version 83.0.4103.61 (Official Build) (64-bit) as well as FireFox 76.0.1 the page http://www.plantuml.com/plantuml/uml/SyfFKj2rKt3CoKnELR1Io4ZDoSa70000 and here the buttons "Submit" "switch layout" do work.
Where do you see the mentioned error? How does it manifest? Please add a screenshot of what you tried.
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