Can’t log/access to my main site

Hi everyone!

I recently switched my client site to a site network.

The main site is :
lafabuleusecantine.fr/ (the father restaurant)

The “sub” site is :
lafabuleusecantine.fr/la-rochelle/ (the new restaurant)

Both sites work but I can no more log or access the main site admin.
The sub site admin is accessible.

I get looped on the main site login page with no message of error.
When I try to access the main site admin from the subsite admin I’m sent to the main site login page.

FYI, the main site is 2 yo, I read on forums that the age of the main site could compromise the network.
Every WP component is up to date and I don’t use any mapping plugin.

Any suggestion?

Thanks!

The page I need help with: [log in to see the link]