I had the same problem happen more than once on different clusters from different sites and networks configurations (with or without proxy/gateway/knox, company network, wifi router, home ISP, public wifi...). With various results It concerned the display of the notes themselves and the welcome page also (sometimes only the notes, the welcome page was ok) Some browser settings block websockets for security reasons. So be aware of that. Sometimes, it can be a firewall issue. Last occurence happened to require a clearing of the outputs of the notes. I was working on a note including a spark job that took ages and the progress bar froze. I restarted zeppelin, Spark, Yarn, nothing seemed to fix it. The note seemed "lost", undisplayable, desperately blank. My fix (after a couple of hours trying to figure what the hell was causing that) was to Hover over the note in the welcome page, and click the rubber, "clear output"... and voila,I was able to open my note again. Note That this has to be done after a restart of the Zeppelin service. Just for note (no pun intented) when I explored the browser prior the fix the websocket kept trying to load content from the server but nothing came out.
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