The 502 bad gateway error means that the server is receiving an invalid response from another server.
The first thing to try is to refresh the page as it may just be a temporary glitch. The next most common solution to this problem is to clear your cache and cookies within your browser history, below is a screengrab of what this looks like on Google Chrome (make sure and set "All time" as the time range). For other browsers just google how to clear the cache on that particular browser.
If it still persist it may be worth trying a different browser to the one you currently are using (Chrome usually works well).
If you still have difficulty get in touch with the library and we will try to find a solution.