Hi nqo,
I was under the impression that Processwire was installed as a Yunohost app, and thought to see if it gives default credentials while installing.
With reference to the request-thread, do I understand correctly that it is not the case?
- nginx is not used especially for Processwire as a reverse proxy. If you have a look in
ls /etc/nginx/conf.dof your Yunohost, you’ll see that there is an nginx-config for each of the installed applications. - Not per se, but it is the preferred web server by the Processwire-folks. They have more than one page with requirements; the list in the ‘download’ section mentions “Apache or compatible”, while the list in the ‘about’ section says “Apache and others, see nect paragraph”: ProcessWire is also able to run on any non-Apache server with a few adjustments.
- I don’t understand the question. I think the way to go is to
- use the ‘My Webapp’-app,
- download Processwire according to their instructions
- match the nginx-config to fit Processwire
Maybe steps 3.1 - 3.3 is just what you did 
In your post you list all the steps that you have done at the front end. Did you also have a look at logs? What happens there?
It is not totally clear to me what does happen in the front end.
There is no error, it just does not log in?
There’s a post on stackoverflow from some five years ago, suggesting to set session.auto_start = false in the php.ini file.
Good luck troubleshooting!
-edit-
PS: I think it is not necessary to @call other users on the forum. Active members will see your post anyway and post a reply if they have time and they feel they can help. Members who don’t visit so often probably won’t see that they are called either.
And, one more edit, on the Processwire-forums someone seems to have somewhat similar(?) problems, and has the Processwire-part resolved in the end.