@Julien-Deloubes This is just a guess about your situation based on my own initial setup of nodebb, but you may be having issues because of linux privileges (i.e. some stuff is done with sudo and some stuff isn't). So, try this and see if it helps:
Go to the directory where your nodebb is installed and run sudo npm install
Next type sudo chown -R youruser:youruser . <-- youruser is your username...
Run ./nodebb setup
Run ./nodebb dev and post any errors or warnings you get here...
To summarize, you'll npm install with sudo because sometimes things need to rebuild which may call out to folders/files requiring su privileges. Then, you change ownership of your entire nodebb installation directory to your username to avoid privilege issues when running ./nodebb. You re-run setup to ensure things are built properly and where they need to be. Finally, ./nodebb dev will give you some useful output as nodebb loads and if you are having routing issues it will probably log those to the console as well. So, run it that way click around, type things and do stuff to see what happens and if you are getting errors/warnings post it here.
Having said all that, I may be completely wrong, but it only takes a minute or two to try out and definitely won't mess things up. Just don't set your port to anything low (i.e.port 80) or you'll have issues because linux reserves the lower port numbers for su privileged users. Basically, stick to port 4567.