Monitoring shows no_data

I have the problem that sometimes the "Monitoring" Tab in Kibana shows me nothing.

Sometimes instead of home?_g=() there is no_data?_g() at the end of that URL. (Sometimes I am even surprised to see "no_data" immediately, without waiting at all for what would seem like a timeout.)

In this case the browser tells me that api/monitoring/v1/clusters and api/spaces/space returned 504 Gateway Time-out.

I have the impression this might be happening when the cluster is too stressed by the volume of the data, and sometimes it can be resolved by adding an extra data node. However at the same time the health of this cluster looks to be in order:

es-health.sh  
epoch      timestamp cluster       status node.total node.data shards pri relo init unassign pending_tasks max_task_wait_time active_shards_percent
1598278267 14:11:07  elasticsearch green          14         9    506 253    0    0        0             0                  -                100.0%

es-api.sh _cat/nodes?h=name,cpu,disk.total,disk.used,disk.used_percent,heap.percent,ram.percent,uptime
elasticsearch-client-5b894b8d85-xhklf  8 199.9gb  14.3gb  7.19 68 93 3.1d
elasticsearch-data-1                  16 502.9gb 299.3gb 59.51 67 97 3.1d
elasticsearch-master-2                23   3.8gb  32.9mb  0.83 34 98 3.1d
elasticsearch-data-4                  18 502.9gb 332.8gb 66.18 52 98 3.1d
elasticsearch-data-8                  29 502.9gb 325.6gb 64.75 42 99 3.1d
elasticsearch-data-7                  23 502.9gb   295gb 58.67 74 99 3.1d
elasticsearch-data-6                   9 502.9gb 291.6gb 57.98 37 95 3.1d
elasticsearch-data-2                  22 502.9gb 309.6gb 61.56 61 99 3.1d
elasticsearch-data-0                  16 502.9gb 308.9gb 61.43 50 98 3.1d
elasticsearch-data-5                  25 502.9gb 338.1gb 67.24 39 98   3d
elasticsearch-master-0                24   3.8gb  32.9mb  0.83 61 98   3d
elasticsearch-master-1                 7   3.8gb  32.9mb  0.83 22 95 2.9d
elasticsearch-client-5b894b8d85-p56b2  8 199.9gb  16.3gb  8.16 71 99 3.1d
elasticsearch-data-3                  23 502.9gb   279gb 55.48 64 98 3.1d

The other Tabs like Discover, Visualize, Dashboards do give me a response, although sometimes after several seconds.

(Elastic Stack Version 6.8.6)

If you're seeing 504 gateway timeouts, then that is related to the network you're operating on. For example, if you are using Kibana in the cloud, then it may be related to the network settings of the cloud provider.