URLs in other languages than english not working

Regards.

I've been hitting my head against a wall with this for the last two hours.

I have a site in english and french. All is happiness in english. For the language switch, I chose the "prefix" method: "fr" for french, none for english.

The rendered pages are correct. The URLs seem correct, the prefix is added where it has to be, the lang switcher block shows existing URLs for translated content...

It's only that the prefixed URLs don't work. I get 404 error instead.

The thing that made me push the "panic button" is that this URL works...

/admin

but this doesn't:

/fr/admin

The admin URL should always work! I even tried to add the language switcher block to the admin panel, so it's Drupal the one who provides the URLs in the other language. No luck.

But, again, the rendered page is for the most part correct. The language switcher block is giving correct URLs, the rendered HTML has the right classes in BODY referring to the language... All correct. It's like Drupal selects the right language... but too late. It's like the URL is being marked as "404" before i18n has the opportunity to examine it.

Does it make sense?