I had a wordpress that was served from domain1.com with permanent redirection from domain1.fr and domain2.fr that is one main domain and two redirections. I then configured a wordpress multisite using domain1.com and subA.domain1.com, subB.domain1.com and some others subdomains.

define( 'WP_ALLOW_MULTISITE', true); //Manually allowing multisite
define( 'MULTISITE', true);
define( 'SUBDOMAIN_INSTALL', true);
define( 'DOMAIN_CURRNT_SITE', 'domain1.com' );
define( 'PATH_CURRENT_SITE', "https://wordpress.stackexchange.com/" );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );
define( 'COOKIE_DOMAIN', $_SERVER['HTTP_POST'] );

It went well and I thougth everything was fine but after double checking, only domain1.com and the subdomains (subA.domain1.com, subB.domain1.com,…) are working. So that the main site is only available from domain1.com and no more from domain1.fr and domain2.fr . The other domains now give back a 500 Error establishing a database connection.

I host on OVH VPS and use a traefik/docker stack if that matters.

Any advices on what to check/do, I can updates with some configs if needed.