All content gone

Hello,

After installing the Polylang plugin I’m using on every single one of my websites, it appears that I encounter a problem, for the first time.

In fact, my website’s content entirely disappears, the frontpage displays the following message : “not found”.

As soon as I deactivate both plugins, everything gets right again.

What should we do ?

Quite urgent,

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