Have you ever been field calculate on Utility Network device feature ?
When i try I get this error.
I think I've had the same error.
Usually when something goes wrong with the UN, I close everything, start a new project, load the service in a new map and redo my action. It often works well, but I don't know why.
Actually, I'm experimenting by recreating the project as you said. It works for many troubles, but it doesn't work for field Calculate.
When I export the device layer (or delete UN feature set) it does not cause any problems. It gives an error when UN is found in the dataset. Maybe this is a bug
You appear to be running a field calculate on a layer that has a join against it. Can you confirm that the field calculate works without the join?
I was able to do this via a file geodatabase using the spatial join tool, add join tool, and calculate field tool against a regular layer. I'd recommend log a case with support with your particular workflow (subtype group layers, services, add spatial join tool) if you're not able to work around it.
If you can't work around the issue and you're comfortable writing python, an alternative (although slower method) would be to use a combination of a arcpy data access search / update cursors to scan the spatial join table and push the edits to the corresponding rows.