Excel error - tricky (does not specify which XML file)

Hi, I'm generating an excel file using php code. My whole spreadsheet looks fine and the formulas are working correctly. But, when I open the spreadsheet, I get "Excel found unreadble content error". When excel repairs the sheet - it just says "Excel completed file level validation and repair. Some parts of this workbook may have been repaired or discarded." - Here, it does not show which xml file it repaired because of which I'm not knowing where is the mistake. I tried opening the file using the "Open and Repair" option , then checked the differences between the xml files. BUT, I couldn't find any. Can someone help me about this ?

Have you tried using Open XML SDK v2.0's "Open XML SDK Productivity Tool"?  This offers a "Validate" function that often provides much better detail of any incorrect formating of the document.

Another option is to use the Open XML SDK v2.0's OpenXmlValidator - this often provides even more detail