To partially answer my own question and continue on what I mentioned in the comments, here is the difference between the Mac and the herohu.htaccess file I need to do:
RewriteRule ^([^/]+)/([^/]+)/([^/]+)/$ $1/$2?arg=$3
This is more a workaround than a solution. I believe that the versions of Apache are different from each other, but in the end I just gave up, debugging it further and use the .htaccess file with two sections in it, which I comment / uncomment.
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