Services see the site's certificate as self-signed (nginx/ubuntu)

[rg305]

And I repeat myself (but you obviously don't understand HTTP/HTTPS):

@rg305

I can only conclude that this NOT an nginx problem, nor an LE cert problem.
Find where the other cert is being served and you have found the problem.

@rg305

localhost:3000 can't possibly be doing HTTP & HTTPS

@rg305

You must verify which of these two works (one will surely fail):
HTTP://localhost:3000
HTTPS://localhost:3000