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VLOOKUP fails due to locale issues
Excel (at least in its german version) seems to sort german umlauts in a different manner than PHPs uasort(). This can cause big trouble when using VLOOKUP with $not_exact_match == true.
AFAIKS, a quick bugfix would be to use uasort() in LookupRef.php, line 653 in any case, regardless of the value of $not_exact_match (besides, the following code does not seem to efficient to me; perhaps some binary search could speed up VLOOKUP references quite a lot).
I have a small test file to reproduce the problem, however the upload does not work right now... I'll try again later on.