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I currently could not reproduce this issue though. No other users have reported similar problem so far as of today also.
It is possible that it was just a temporary hiccup due to network/internet connection lag.
Can you please retry again? Try also with other browser, like Chrome or Firefox.
I already test on my end using your form http://www.jotform.co/form/52672556055863 and the Form Designer loads just fine.
Hope this help. Let us know if the issue remains.