Thanks for your message. We indeed used OSI definitions as a basis. I even suggested taking it verbatim, but during the discussions the current definition emerged somehow.
I am currently writing the ordinance for applying the law and I indent of putting EUPL as the default recommended license (as it’s GPL-compatible), as well as a list of permitted licenses — normally the particular licenses are defined in ordinances, rather than the law itself. I will let you know once that’s ready.