Using Windows XP and Office2003:

In column AJ we have a formula
....
=CONCATENATE(A97;G97;H97)
=CONCATENATE(A98;G98;H98)
....

the result of this is either 14 or 15 characters (depending on the month of
the year)

in column AT we do a COUNTIF:
....
=COUNTIF($AJ$83:$AJ$4916;AJ97)
=COUNTIF($AJ$83:$AJ$4916;AJ98)

since all of the values in the AJ column are unique, all of these formulas
should have "1" as a result, however, the ones with a string in the AJ column
of 15 characters give "3" as a result (conclusion: that's for months 10, 11
and 12 where the 0, 1 and 2 aren't recognized/counted).

This would mean that only the first 14 characters are evaluated. I've tried
several workarounds, but in itself these functions should work (or am
I missing an elephant here).

I've also seen questions about formulas that only work when they refer to
cells with straight numbers instead of formulas, but even when I replace the
'concatenate' results with the resulting value of the cell, the result stays
the same.

For another formula on another sheet to work we need to have this error
sorted out, and so far I haven't found what causes this. Can you point me in
the right direction?