Yes, I'm trying to overwrite a hosted feature layer, the service definition file is present in AGOL.
I'm not sure this is the problem, however, because I did try to create a completely new hosted feature layer and I got the same error. I even created an entirely new .gdb, created an entirely empty featureclass, gave it a single geometry and attribute, and tried to share it as a hosted feature service using an entirely new .mxd and I also got the same error message..
I recently installed ArcGIS 10.5, I wonder if that has something to do with it?