I'm getting this error when trying to add individual /FeatureServer/5 id of the layers to a web map in the new map viewer.
to get some basics out of the way -
1. Yes the layer works - you can add it via the classic viewer just fine. This only happens in the new viewer.
2. The layer comes from an arcgis enterprise deployment - and we add it to AGO while enabling storing the credentials.
The difference between the new and old map viewer is that the new map viewer makes some extra requests to a "checkUrl" service.After that - the requests to the service do not include the token so they fail, probably leading to the error above: