I am home Synology NAS owner, and I am a long time Let's Encrypt Certificate user, has no problem with it. Today I have found out my certificate was expired, although it was always automatically updated. Have tried to update it manually from Synology DSM - failed with message on the screenshot below... My domain is sergeskor.org, managed by Cloudflare. Domain is accessible, ports 80 and 443 are open and forwarded to my NAS.
What may be the problem?
Welcome to the Let's Encrypt.
Are you expecting an nginx server to respond on that domain?
$ curl -ksvo /dev/null https://sergeskor.org/
< snip >
< HTTP/2 200
< server: nginx
< date: Sun, 24 Aug 2025 18:33:56 GMT
< content-type: text/html; charset=UTF-8
< vary: Accept-Encoding
< vary: Accept-Encoding
I believe it should be Apache... According to Synology settings is Apache 2.4...
Ok, I have changed it to nginx and immediately back to Apache, and try to update certificate from Synology DSM, - boom, it has been updated... Seems like Synology glitch?
Anyway - thank you so much! Will watch it...
Thank you again for help! The problem was really un-expected for me...
May I ask few questions to educate myself? 
In general - do certificates works with nginx web-server? Why have you check it first?
What is the simplest way to check what software is in use as web-server from client side?
Yes, they work in a broad array of servers that support TLS (SSL)
Make an HTTP(S) request to that domain and look at the response headers. Many ways but one example
curl -i http://example.com
I'm not sure why @linkp thought to look there first but we commonly make such requests when reviewing a trouble report.
I actully looked there second. 
It was after testing at letsdebug.net passed.
The curl command I used is visible in my earlier post.
Don't forget to run your tests from the public internet, and not your LAN. You are troubleshooting why traffic from the internet is not reaching the expected destination. If the host is on your LAN, internet traffic tends to take a different route than your LAN traffic follows.
Thank you for explanation and for the link to letsdebug.net.
Can you please check one more time? I am trying from my job place and I see also "Server: nginx" :-), although https connection is OK, and I see certificate is also ok and renewed. What should I see for Apache server?
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