Looks like port 80 is not released by whichever process was running earlier.

So, kill that process with:

sudo fuser -k 80/tcp

Then restart Nginx.

Curiously I’ve seen this “backlog 511” error in Plesk. Do you use Plesk? If so, switch off and on Nginx by going to Tools & Settings > Services Management.

If that didn’t work, tell us exactly what password you changed.