Error handling services request :Wait timeout for the request exceeded. Code 500

Error Message

When navigating to the ArcGIS Server REST endpoint and selecting one of the published services, being it a feature or map service, the following error is returned:

Error: Error handling services request :Wait timeout for the request exceeded. Code 500

When adding the same item to Map Viewer, the following error popup displays:

Unable to add layer . Timeout exceeded"

Cause

This issue was observed to happen on initial machine boot-up and happens with single-machine deployments. When the machine is first started, there are no ArcSOC processes to handle requests for the services. The shared instance pool produces no ArcSOCs, but the dedicated services produce ArcSOCs on cold start when they receive usage.

This issue is not consistent, as it is most probably environment specific.

Solution or Workaround

Restart only the ArcGIS Server Windows service. This creates the required ArcSOC processes for the shared instance pool, allowing services using this shared pool to receive requests again.