The ssl certificate on subdomains stopped working

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Kindly, may I suggest below two articles for troubleshooting SSL on your sub-domains:

Deprecated - SSL/TLS not working on subdomain

This tutorial is deprecated in favour of Troubleshooting · Cloudflare SSL/TLS docs

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Please note, this tutorial post contains links to external sites. I am not endorsing any external links. This tutorial post covers the first steps you should take if SSL is working on your main domain, but not on a subdomain. 1. Is there a DNS record for it? Check the DNS app in your Cloudflare dashboard to see if there is a DNS record there for that subdomain. 2. Check that the DNS record is working co…

Deprecated - Subdomain too deep

This tutorial is deprecated in favour of Fix VERSION_OR_CIPHER_MISMATCH · Cloudflare SSL/TLS docs

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This tutorial covers a possible reason for the SSL_ERROR_NO_CYPHER_OVERLAP and ERR_SSL_VERSION_OR_CIPHER_MISMATCH errors (Firefox and Chrome respectively) when seen on a subdomain. The Cloudflare universal certificates cover example.com and *.example.com. This means that it covers any subdomain one level below the domain you signed up with. It will cover www.example.com and subdomain.exampl…

Before moving to Cloudflare, was your Website working over HTTPS connection?

Nevertheless, may I suggest you to check your SSL certificate validity and in case it’s expired, kindly renew it.

Furthermore, if you used Let’s Encrypt or some other, before doing this, kindly enable the “Pause Cloudflare for this site” option from the Cloudflare Dashboard.
After renewing the SSL certificate at your server and testing out, make sure your Website works over HTTPS.
Upon success, un-pause and make sure your DNS records are proxied (as they were).
Do not forget to set the SSL/TLS option to the correct one → Full (Strict) SSL .

Why you should choose Full Strict, and only Full Strict

Unencrypted & unverified connections Imagine you open Paypal and suddenly get that warning [image] Would you continue? Probably not. For decades leaders in IT security have advocated that people upgrade their sites from unencrypted HTTP to secure HTTPS. And for a reason, everything you send via an HTTP connection is sent in plain text and can be intercepted at any point between you and the server. Equally, you’d probably not proceed if you got such a warning, right? [image] That’s when th…

In case you do not have an SSL certificate, you can use Cloudflare SSL, if so, kindly make sure you follow the instructions as follows on the below article to set up an SSL certificate using Cloudflare Origin CA Certificate:

• Origin CA certificates · Cloudflare SSL/TLS docs

Here is a way to re-check if you correctly setup the SSL for your domain with Cloudflare:

Deprecated - Step 2: Setting up SSL with Cloudflare

This tutorial is deprecated in favour of Get started with SSL/TLS · Cloudflare SSL/TLS docs

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This tutorial covers getting SSL working with Cloudflare in various different scenarios. This assumes you already have your website set up on Cloudflare with all your DNS records set to , if not - please visit <a href="https://community.cloudflare.com/t/64309">Step 1</a>. Do you want the website to use HTTPS? Yes Do you already have a valid SSL certificate installed…

@user20143

the certificate is active until March

May I ask where from did you get this information?
Is stated from Cloudflare Dashboard under the SSL/TLS tab → Edge Certificates → Unviersal SSL or?