I also just encountered this error. I previously shared this map from ArcGIS Pro 2.7 to ArcGIS Online without issues. I have since made edits in Pro including adding fields to a layer, changing the symbology of the same layer, appending features to a different layer, and removing a layer. Everything else is the same. Any help on this is greatly appreciated.
Try going into "C:\Users\<username>\AppData\Local\ESRI\ArcGISPro\Staging\<connection_name>" and delete all .sddraft files you find there. We had a lot of those files named the same with only different numbers in brackets. It helped when we deleted them all. The publishing started working again.