JSON error might be caused by the system's background process, sales80.
While 502 Bad Gateway is an HTTP status code which means that one server on the internet received an invalid response from another server.
Let's try to use a private browser and check if you're able to access QuickBooks there.
If you're able to access QB, go back to your regular browser and clear cache. This is to refresh the server and boost the browser's performance. You can also try to use a different browser.
Let me know if there's anything else I can be of help.