My home institution is registered by default (and it's not the right one)

When accessing to the online resources, you are asked to select your institution, in this case "University of Rennes 1" :

Choix de l'établissement

This setting may have been saved to your computer station by default. No problem if the University of Rennes 1 has been registered, but if it is another institution (INSA, ENS ...), you are blocked!

What is happening and what to do?

You can first go to the address https://wayf.univ-rennes1.fr/WAYF to reset your institution by default.

If that does not work, the faulty may be Zotero, the bibliographic data management software.
Go to your Zotero configuration to delete the configured proxy server, and for safety, remove the cache and cookies from your browser.

If you do not control the Zotero settings, a user manual in Firefox:

Serveur mandataire Zotero

If the problem persists, you can deposit a ticket.