In fact, clicking on any link (links from a sub-page to its main page, buttons...) in the headers (in blue) does not work.

A workaround has been found by @Yug (see the git-hub discussion).
He added the piece of code to the MediaWiki:Common.css page on LiLi as a temporary solution.

@Yug In my opinion, the issue shouldn't be closed until the actual code is fixed (even if we identified the problem and found a workaround). The piece of code that you added in MediaWiki:Common.css is temporary (waiting for the actual code to be updated), isn't it?

Edit: I'll change the title for the former one, because the problem was concerning not only the "Start a new discussion button" but every pointer event on the texts contained in blue banners (including description, page names, and breadcrumbs)

@WikiLucas00, @Pamputt : this task title is "Start a new discussion in Chat room (and any link in any header) not working". Lingualibre.org website does not presents the bug anymore : the mentioned bug has been fixed and resolved. This task title has been answered.

It now moves to code implementation, aka Github :

• BlueLL/#5, open 4 days ago.

Planned steps on github:

• CSS code review
• Check side effects, refactor css, git push
• Deployment by Michael on Wikimedia server.

This task, "Start a new discussion" in Chat room (and any link in any header) not working" can be close so I may focus on the next steps and not be dragged back managing upstream tasks. Our volunteer cognitive resources should now move on following steps. The ball has rolled to next step. Keeping open a zombie tasks is counter productive.