When packaging areas of a web map for download (offline use) in the Field Maps application from an ArcGIS Enterprise portal, the packaging fails with a generic error message 'Failed', as shown in the image below.
When viewing the network traffic in the browser the following error message is seen:
Failed to add item error: Portal request addItem failed with error ERROR: code: 400, Invalid 'dataUrl' parameter, Bad syntax in request.
The web map and underlying data is configured as required for offline use, and no errors or warnings are logged in the portal or server logs.
The HyperText Transfer Protocol (HTTP) 400 Bad Request response status code indicates that the server cannot or will not process the request due to something that is perceived to be a client error.
Viewing the network traffic when the packaging fails, it's observed that the 400 response is returned from the Sharing/Proxy endpoint.
By default, the portal's proxy capability is unrestricted, however it is possible to restrict the portal's proxy capability by defining a list of approved web addresses. The ArcGIS Enterprise portal is then only able to proxy requests to the addresses in the list; all others are blocked.
Viewing the security configuration of portal from the REST admin endpoint, the 'allowedProxyHosts' was configured, but the internal domain (xxxxxx.local) was not added to the list of allowed hosts.
Update the allowedProxyHosts security configuration parameter from the portal REST admin endpoint to include the internal domain in the list of web addresses to proxy requests. As in this example and shown in the image below: