Finding and removing an old certificate

[Alasdair McAndrew]

Hello,

Sometime ago I hosted a nextcloud instance on my VPS, certificates being
provided by letsencrypt.  I don't run it any more.

However, just recently, I have been getting popup messages on my local
computer saying:

--
Cannot connect securely to nextcloud.mysite.net:
The host name did not match any of the valid hosts for this certificate
with Certificate mysite.net
Organization: <not specified>
Unit: <not specified>
Country: <not specified>
Fingerprint (SHA-256):
55:24:e8:fe:46:7e:4e:a5:1f:b2:bb:67:a5:dd:76:ca:28:73:38:00:03:71:ba:4e:60:09:3e:bc:8c:02:3a:b1
Fingerprint (SHA-512):
fb:84:1c:9b:24:b5:84:bf:eb:27:e7:4f:75:e8:ef:d0:6d:dd:3e:ab:4a:e2:70:e2:8d:83:cd:34:31:90:a0:f0:aa:8b:3c:6f:51:4e:12:f0:df:0d:ae:68:59:6f:c1:75:f7:1f:40:87:00:82:ff:dc:1d:e3:99:8a:9f:be:8a:3e

Effective Date: Mon Feb 5 00:25:06 2024 GMT
Expiration Date: Sun May 5 00:25:05 2024 GMT
Issuer: R3
Organization: Let's Encrypt
Unit:
Country: US
--

The only way I can get rid of this message is to check the box "Trust this
certificate anyway".  I'm not quite sure what I've done that is producing
this behaviour, and whether it's related to my home machine (which runs a
very recently upgraded Arch Linux), or my VPS, which runs the server
version of Ubuntu 20.04 LTS (so without a desktop).

The fingerprint above is that of cert.pem and fullchain.pem, in a folder on
my VPS managed by SWAG, a proxy server from linuxserver.io - and only
recently installed.  However, nextcloud is not one of the subdomains I have
listed to be managed.  So I'm completely unsure why these popups are
happening.  I haven't used nextcloud in more than a year, but this
behaviour is only just starting.

How do I stop these annoying popups?

Many thanks for advice!

Alasdair
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