I remembered where I saw something similar: Bizarre problem with bulk import
Despite the error message being the same, this is a separate issue.
I checked the spreadsheets and there’s nothing different in how file 10 is set up from file 6 and 7. It also looks like they used Chrome in their video, so I tried uploading file 6 and 7 in Chrome and wasn’t able to replicate the error there, either.
Despite not seeing any formatting differences between files, sometimes if there’s hidden formatting quirks in an Excel file, it helps to save it as a CSV then convert that CSV back to an Excel file. I’m going to send you converted files for your user to try and see if they work.
If that doesn’t help, clearing cache and cookies can sometimes fix issues with websites behaving strangely. They can also try the import in a different browser.