An expression containing a set analysis expression usually - as in your case as well - relies on quotes (e.g. apostrophes) to delimit string values. Your vRegion variable will be expanded inside such a string with delimiters. Unfortunately, one or more regions contain quotes themselves and will break your expression. This is what the front of your expression gets turned into when selecting region "Provence Alpes Côtes d'Azur":
=num(only({<Région={'$(=Provence Alpes Côtes d'Azur)'},[Type d'indicateur]={'Usage'},Service={'DCC'},
[Libellé de l'indicateur]= {'% de fiches RCP '}>} [Résultats semestre 1 - 2017]) ,'##,##%')
The string modifier for field Région now terminates in the middle of d'Azur...
