Squarespace - Custom static page "malformed data"

Question

I built a small single page application as a static page. I was able to upload it and activate it on my staging site a little while ago. After uploading the code to my production site, I cannot find the static page. I checked on my staging site and it is telling me:

This page has malformed data and can not be edited. This error has been logged. Please contact customer care if this issue persists.

Why is the page not working? I followed the instructions exactly, and it was working before, but now it doesn't even show up as an option.

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I posted this question to Squarespace Answers, but I don't have much hope of getting help there, so I'm posting here also.