Trying to import an Excel file in as SharePoint 2010 List resulted in the error:
The specified file is not a valid spreadsheet or contains no data to import
Double-checked the spreadsheet for formula errors, consistency etc. everything looked good but still no importo. The solution is to go to Internet Explorer > Internet Options > Security > Trusted Sites and add your SharePoint site to the list of trusted sites. You can use a *. to avoid having to list multiple subdomains, e.g. “https://*.mySharePointSite.com”
Be sure to disable the HTTPS checkbox if it’s not a requirement for your deployment.