Check that your file really is an Excel xlsx file and not some file that only has a .xlsx extension, but is really something totally different. Try opening it with Excel to make sure it's not corrupt. If you can open then then save it explicitly as an ooxml file with an .xlsx extension.
Ok, could be. But my bet is that the application that exports it as xlsx doesn't do it properly and creates an xlsx file that does not conform 100% to the ooxml specification. Feel free to open an issue with qlik support.