I’ve seen this before a few times. Connect is a very basic ETL program (extract, transform, load). I think Spider has kept it that way because It really helps with perfomance if there are fewer bells and whistles.
The downside of this is that the spreadsheets have to be ‘clean’. That is, they should only contain row and column data. Calculations are fine, but things like Pivot Tables or Images seem to cause Connect a real problem and it does not provide a helpful warning just the Loading… message.
Check your spreadsheet and remove anything that is not required for the upload.