Wrapping the whole line in quotation marks makes it work:

-addonsDir "C:\Users\patri\Documents\My Games\ArmaReforger\addons,C:\Users\patri\Documents\My Games\ArmaReforgerWorkbench\addons,C:\Users\patri\Downloads\s4-reforger-mods\addons"

This is however not standard expected behaviour if I am not mistaken. In any case it should work both ways.