I am facing issues with the ExB developer edition. All the layers in the map doesn't load and in map Layer Widget we see an error: "Error occurred while loading this layer" .
The same web map loads in Map viewer and even loads in ExB created through portal. However, it doesn't work in dev edition.
Not sure if this is your case, but I found a similar post: https://community.esri.com/t5/arcgis-online-questions/existing-layers-showing-quot-error-occurred/td...
More details would help locate the problem.
@Allen_Zhang , thank you for the help. it was internal CORS issue resolved by our IT by adding CORS to header or something