Post this in Report an Issue. (See tab at top of this screen) If you would like the FileMaker Techs to examine a file or files that you have, upload them to a file sharing site and then post the download link as part of your issue report.
PS. have you tried defining two ranges for your spreadsheet and importing one range at a time to import this data in two pieces? Not only might that be a work around, but it might identify the exact issue causing problems for you.
I haven't but I will try, if it does not work I will report an issue. Does filemaker have a specific download site for this purpose?
I'd report this in any case unless you find a specific issue with your excel file. It's up to you to pick an upload site and use it for any such demo files.