I've never seen it before, actually looking into it a bit more, that error is from ES: eg Elasticsearch 7.0.1 - Trying to create too many scroll contexts. Must be less than or equal to: [500] .. so presumably your client was using up all the scroll contexts and via some unknown mechanism this caused errors in Kibana.
There isn't sufficient information logged currently to tell if that's related to the loss of availability of Kibana unfortunately. But at least this suggests that you might want to have a look into your client behavior?