I'd advise reducing your import spreadsheet columns to the bare minimum; that will help you to isolate which column has the problem.
e.g. instead of import a spreadsheet with columns a to e, create a workbook with just column a then import, then add column b and then import again, then column c, and import again....and so on and so forth until you find which column has the probem - you could then isolate it to a cell inside the column using the same approach.