I think this has to do with different abbreviations being used for numberformats in different locales. It appears that while the numberformat
dd.mm.yy was valid for range-objects, when I tried to use that range as the sourcedata for the x-axis of my chart, the numberformat got carried over where
yy was not a valid abbreviation for the numbers in the chart. Because of this, it was interpreted as a string and displayed as such.
One quick fix I found was closing and opening the workbook again, which often fixed the interpretation of the numberformat.
Alternately one can go to
format axis > axis options > number > type and set it to the desired format:
If you are generating your charts using vba-code, I found that adding the line
WorkSheets(1).ChartObjects(1).Chart.Axes(xlCategory, xlPrimary).TickLabels.NumberFormat = "dd.mm.yy"
after the data has been added to the chart, fixed the problem just fine as well (go figure).