I have managed to deploy my MERN app on a VPS successfully and serve it with greenlock-express.
But, all of the sudden it stopped working and this is what gets logged when I serve the app:

node server.js

Listening on 0.0.0.0:80 for ACME challenges, and redirecting to HTTPS

Listening on 0.0.0.0:443 for secure traffic

Ready to Serve:

domain_name.com

(node:13453) DeprecationWarning: collection.ensureIndex is deprecated.
Use createIndexes instead.

MongoDB connected!

debug: ignoring servername "www.domain_name.com"

   (it's probably either missing from your config, or a bot)

set greenlockOptions.notify to override the default logger

servername_unknown (more info available: servername)

ACME Directory URL: https://acme-v02.api.letsencrypt.org/directory

By using this software you ([email protected]) are agreeing
to the following:

ACME Subscriber Agreement:
https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf

Greenlock/ACME.js Terms of Use: https://rootprojects.org/legal/#terms

certificate_order (more info available: account subject altnames
challengeTypes)

challenge_select (more info available: altname type dnsHost
keyAuthorization)

challenge_status domain_name.com pending

challenge_status domain_name.com valid

certificate_status domain_name.com valid

cert_issue (more info available: renewAt subject altnames)

When I try to connect to my website on the browser, this is what I see:

This site can’t be reached

www.domain_name.com unexpectedly closed the connection.

Try:

Checking the connection

Checking the proxy and the firewall

ERR_CONNECTION_CLOSED

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I also noticed that when I enter the greenlock.d folder generated when I run the config command:

npx greenlock add --subject domain_name.com --altnames domain_name.com

I only find these two files:

Whereas usually after I load the website the first time after the config, I find all these files: