Chooser dropins.js Uncaught TypeError: JSON.parse is not a function

From the error message, it seems that the issue is that JSON.parse is not set to a function. The Dropbox Chooser functionality does use JSON.parse, and that should be available built-in to the web browser as that's a standard function included in web browsers.

I can't offer support for mootools or any other particular third party library in particular, as it's not made by Dropbox. If there's something that's removing or replacing JSON.parse though, I suggest refer to the documentation for it to see how you can avoid that, or otherwise not use whatever is doing so.