I have a number of simple formulae of the form:
=HLOOKUP(EOMONTH(DATEVALUE("31/12/2011"),COLUMN(A1)),$C$2:$ND$49,ROW(A37),FALSE)

Regularly, when I open the workbook, this block of formulae display the #VALUE error. There appears to be nothing wrong with the formulae, however. If I select any such cell and press F2, then hit Enter again (making NO changes), the formula returns the right result again.

I then have to do this in each row and copy the formulae across again and it works fine.

Any suggestions on what causes this? Something to do with the two date functions perhaps (these seem to cause trouble in Excel generally)?