Compatibility Issue with MS Excel 2013

[Exceloholic]

I used to prepare dashboard in Excel 2007, just got 2013 and added one month's data into it, wherein I did not use any unique feature of Excel 13, this worked well on Laptop. Problem occurred after publishing it worldwide, my colleagues who have Excel 2007, they are getting following error "Excel Found Unreadable Content in "File Name"....". after clicking yes it seems that file is getting opened with repaired content and got below error message -

Removed Part: /xl/pivotTables/pivotTable1.xml part. (PivotTable view)
Removed Part: /xl/pivotTables/pivotTable2.xml part. (PivotTable view)
Removed Part: /xl/comments1.xml part with XML error. (Comments) Load error. Line 2, column 177.
Repaired Part: /xl/workbook.xml part with XML error. (Workbook) Load error. Line 2, column 635.

I do not care for pivots...Problem is unexpectedly entire Name Manager section is empty, my old defined names got removed which should not happen even if I am using 2013 instead of 2007 because this nothing to do with excel version.

I am not an expert in troubleshooting such problems, Need expert suggestion/help.

Many Thanks,
Bijan