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The page was loaded over https: and the page was requesting things over http:. Found it in console. Still an issue I believe. Even though I got it to work.
I cloned your form and tried it on my end using https://www.jotform.com and I was not able to duplicate the error. I was able to access the Designer Mode of the form. See screenshot below.
Although I did notice a warning regarding the mixed content of the site but still I was able to access the Form Designer. May I know if you have any anti-virus in your computer that may block our site?
We will wait for your response.