Recently I discovered that, in certain circumstances, some Kibana .js resources are blocked from loading by popular ad blockers. As an example, this entry in the commonly used EasyList ad block list can trigger on Kibana instances which happen to have the word "analytics" somewhere in the domain name where the the instance is hosted.
When this occurs, Kibana will fail to load display an error message:
Elastic did not load properly. Check the server output for more information.
It's possible to confirm that and ad blocker browser extension is the culprit by inspecting the network pane in the browser dev tools and looking for .js files which were blocked from loading. Turning off the adblocker or otherwise adding and exception and then reloading the page is sufficient to fix the issue.
Others on the internet have also this seen this behavior, as mentioned in Stack Overflow at single sign on - Getting "Elastic did not load properly. Check the server output for more information." when accessing through gatekeeper - Stack Overflow.
I'm creating this topic to record the behavior in case it's helpful for future troubleshooters. Thanks!