Some of my clients imports large Excel files and create custom SharePoint lists.
But suddently they got an error message: “The specified file is not a valid spreadsheet or contains no data to import.”.
What now? Before you spend a lot of time to evaluate the Excel file (macros, functions or s.th.), crawling server logs and sniffing your network. Try to add the SharePoint Site as thrusted site. After that you will be able to import your data you need. Try it 🙂