The forum is https://forum.mubi.pt and firstly I thought this was a problem of the forum. I reported it on itself, but it is inside a closed category, not available to non-members.
Then I thought it was a browser issue. I reported it on Can't see 2. column on a forum site | Vivaldi Forum . The whole situation is described here, so I will not have the work again to repeat everything I wrote there, you can read it.
Now, I am thinking this may be a Discourse issue. Can someone please check?