I am also having this issue. No problems earlier in the week, but starting last night can't upload anything. Also gets stuck on "Uploading service definition", although in my case after about 20 - 30 minutes it'll eventually fail with
StatusMessage: Publishing web layer (AGO)
StatusMessage: Publishing web layer failed (AGO)
StatusMessage: Publishing tool execution failed
Status: Failed ErrorMessage: Failed to publish web layer
Layers that uploaded fine a few days ago no longer work. I am an administrator for my org so definitely have the right permissions, sufficient storage space, everything fine in system health, same problem on different PCs and ArcGIS versions (Pro 2.6 and 2.9), same issue with feature and tile layers. I tried saving a layer as a Service Definition, then creating a layer from that manually through the ArcGIS Online web interface, but that gets stuck on "Checking status...".
Can't think of anything else to try, seeing someone else with the same issue makes me think it's something on Esri's end?