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It seems that Google Sheets expects a semicolon instead of a comma as separator in the COUNTIFS function.
My working formula is: =COUNTIFS('2017'!$T$2:$T$26411;">"&$B$6-$B$10;'2017'!$T$2:$T$26411;"<"&$B$6+$B$10)/$B$1
Thanks Benjamin, I was writing it over and over again and wondering why is it not working :D