I'm having a very similar issue. Some of my layers are not publishing and I get the same error message on my older feature layer. I have no explanation yet. I hope to figure it out!
Mine is still not working and now it is replacing the symbology with other symbols in the map viewer. Some of my collegues are not able to access it too. I'm having a whole host of issues.
I'm having intermittent issues with adding data to ArcGIS online today. For a while I couldn't add data either with Add Item or by publishing a hosted feature service from ArcMap, but all of a sudden I'm able to publish the exact same data with no issues. The ArcGIS Online health dashboard says that hosted feature services are having "intermittent issues," so I'm hoping it's just a temporary problem.