When in opened in Excel, there is three tabs (lower left corner). Close the two empty tabs (Sheet2 and Sheet3) , and see if it works. It worked for me after I removed the two empty tabs.

Also tested some working xls and xlsx blueprints. After I added extra tabs to them and then they didn't work either (Error in qfconvert, could not read xlsx file etc), and end then removed the extra tabs and they worked again.