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Last tested: Apr 24, 2020
This is probably either due to an intermittent outage on the load balancer, or because an incomplete Javascript chunk has been cached on the browser.
The only network requests that are sent while that loading bar is animating are for fetching the JS code that's responsible for things like rendering UI elements and visualizations (these are the "chunks" that you see in the network tab as a page loads). Therefore if fetching the chunks goes wrong somehow, you could get stuck on that page with the loading bar partially filled.
Possible solutions to this are basically to bust the browser cache: hard refresh of the browser (cmd+shift+r) or enable the CDN, which will pull new resources since they'll be coming from a new URL.
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