This error message has been in existence for years (well okay in recent memory which is preatty good):)

Adagio exports to Excel v2.x (or some very old version of Excel). When Excel opens the file it no longer support this version and that is the error message it displays. Not sure what might be lost, I've never found anything missing.

This version of Excel has a record limitation (16,000 I think). If you export more records then these are lost. The solution is to either use GridView (hich uses a current version of Excel) or to do the exports in logical chunks as Ralph suggests.