Microsoft Released a Windows Update : KB3178690 on March 14 which made Excel instable when doing calculations , they released a fix yesterday recently to fix this: https://support.microsoft.com/en-us/help/3191855/march-28-2017-update-for-excel-2010-kb3191855
Tag: Excel 2010
filename.xls cannot be saved in the current format. To save your changes, click OK, then save it as the latest format.
OK , so when you save a .xls file in 2010 from 2003 , explorer bugs you with : “filename.xls cannot be saved in the current format. […]