Editing my #ArcGISServer layers from #QGIS is *usually* a wonderful experience, but for anyone looking to do the same:
1. Run "force right-hand-rule"
2. Run "repair geometries"
Use Q's incredible "edit in place" mode in the processing panel.
Now you can edit!
No idea why, but my #Esri layers routinely have invalid geometry that must be fixed before edits from Q will go through.