This is a point shapefile that I just made in ArcGIS desktop 10.0 from a .csv. I get the exact same error message as the OP. My first try had no .prj, so I made a version in WGS_1984_Web_Mercator, but I got the same result (which is to say no result).
As a side note, I have Adobe Illustrator installed on my machine, and it renames the .sbx files as Illustrator Tsume files on my local machine. I wouldn't think that would interfere with ArcGIS online, but I'm tossing that fact out there just in case...