Hi Matthew,
Just to provide you more details: I created something like more than 500 shapefiles, like the one in the sample, from a table in R statistical software. Apparently, I have problems with all of them and I don't know why, since these files work just fine on my ArcMap.
I also tried to upload some of them using a file geodatabase, but the result is the same. Actually, my goal was to do a stress test with file geodatabase in ArcGIS Online in order to understand how many layers I can put in them before something goes wrong.
Thanks again for your suggestions,
Salvatore