It took me a while to get through this.
I reinstalled the server, retried the docker way. Restarted again tried the source way. It was still not working.
I finally found the issue. Somehow the install tutorial sets taiga service (taiga-back) on port 8000 where I do not know why the front interface tried to check it on port 8000.
I remapped the service to 8000, changed nginx configuration to also point the API to 8000 and now it works.
Finally to explain to newbies like me that may read this post, your first line of debug is not the server, it is the console of your browser...