It's a bug that I thought had been fixed, caused by uppercase in the JID in your profile. ClementP doesn't match clementp and it prevents you from using your name.
Can you try re-saving https://api.nantes.happy-dev.fr/projects/5/ and see if it helps?
I just created components/solid-xmpp-chat#205 (closed) to fix this and related issues from happening in the future. I have nothing else to work on today, so... :)
There are also some other problematic users I see in that circle. I look forward to rolling out components/solid-xmpp-chat#188 (closed) so we can get things like that solved and fixed.