Multi domain mapping

I have a domain (cgtairfrance.fr) mapped to a sub site (http://reference-syndicale.fr/cgtairfrance) and it works fine on some articles : http://cgtairfrance.fr/2018/11/la-cgt-ne-signera-pas-laccord-salarial/ but the home page or certain categories lead to the network signup page. I've tried changing permalinks, clearing cache, disabling plugins. I have no idea where this comes from.

It used to work fine, but i had to deactivate all plugins, and reactivate plugins one by one (due to performance issues). And now i have this problem. Any leads ?