Ok thank you @gotwf
Also looks like I spoke too soon - regarding your original question 1, if I bypass nginx and connect directly using the server external ip to port 4567, I can see nodebb launch "correctly". But if I access it through the domain name, it gives me the above result. So that did matter.
Here is the nginx server block (file name - forum.kado.ai.conf)-
Here is the nodebb config file.
@PitaJ What kind of reverse proxy configuration issue may it be? I've tried to change some nginx configuration lines, but withou result.
Oh I forget, I have used certbot to install SSL for my NodeBB forum, it changes some configuration in the nginx file. I do it according to this: https://medium.com/@hariomvashisth/free-https-within-5-minutes-5b67479849fa
I notice that on the mobile safari browser, after click the 'go' button, the url seems to stop for 1 or 2 times, maybe caused by http/https transformation. But not very clear, any idea? Thanks!