What am I missing here?: The first link works on a server with 10.3.1 on it and returns an image - it is a raster dataset in ArcSDE. The second link is 10.6 on a different server that is in production, mosaic dataset and throws an error - <ServiceException code="InvalidLayers">
Parameter 'Layers' contains invalid values.
Everything in the URLs are the same as are the setting on the services the best I can tell. Ideas?
In your first link you have https://server
In the second on there is https://gisimg.co.dakota.mn.us
I entered both of these and both worked:
The first server is an internal server of an earlier version of Server and I am hiding the name in the example.
You tried the full URL I provided and didn't get an error, but got a 256 by 256 image returned to the browser?
It looks like the section "Specifying Custom Parameters" is new in the latest documentation and not found in 10.3, perhaps a clue in the Notes section. http://desktop.arcgis.com/en/arcmap/latest/map/web-maps-and-services/using-wms-service-layers.htm