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Hi Artur,
According tothis article,it mentions that All example formulas in this topic use commas "," as the parameter delimiter character. In some countries, the comma is reserved for use as the decimal mark. In such countries, users creating a calculated field must use semi-colons ";" as the delimiter character.
I think that German is the country which needs to use semi-colons ";" to replace commas "," in the formula. I also check this point on my site. I find it works when I change commas into semi-colons in the formula: =TEXT([Column2]-[Column1];"h:mm")
Hope this is clear for the situation. Feel free to let us know if it works fine on your end.
Best Regards,
Eli