Excel 2003 & 2007 formula compatibility
 

I have a user who built a big spreadsheet in Excel 2003. This spreadsheet
contains many formulas that depend on many cells. When the spreadsheet,
created in Excel 2003, is opened in Excel 2007, many of the cells say #NA.
When one looks at the formula in Excel 2007, the formula states =#NA instead
of the actual formula. This creates of chain of many cells displaying #NA.

For example, the formula for 2003 is: =EOMONTH(cell number)
formula for 2007: =#NA

Trying to trace the error gives me: The Error Tracer encountered existing
arrows or a circular reference. Delete all tracer arrows or resolve the
circular reference, and then choose Trace Error again.

Now if the same spreadsheet is opened in another computer in 2003,
everything is fine. If one retypes the same formula in 2007, everything will
also be okay.

Is there a known compatibility issue between Excel 2003 and 2007? Is there
something that I am missing?

Thanks!