When opening an export of the responses in Excel 2016 the following error message pops up:

"Excel cannot open the file 'filename.xlsx' because the file format for the file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."

Editing the file in Notepad++ shows white lines at the start of the file and missing closing tags at the end of the file. Removing the white line at the start of the file results in Excel trying to repair the file (which eventually works).