If it doesn't go away on its own, it is usually a sign that your internet provider or antivirus solution is tampering with the connection to download.openmpt.org, presumably becacuse they think OpenMPT is malware. More information can be found in this thread: https://forum.openmpt.org/index.php?topic=7250.0

you have to update the page to support at least TLS 1.2, according to this - https://support.mozilla.org/en-US/kb/secure-connection-failed-firefox-did-not-connect?as=u&utm_source=inproduct#w_secure-connection-failed
and this too:
https://hacks.mozilla.org/2020/02/its-the-boot-for-tls-1-0-and-tls-1-1/

If our server didn't support TLS 1.2, no web browser in this world would be able to connect to the website, not just yours. And in fact you are connecting to the very same web server successfully as you are able to write a bug report here: bugs.openmpt.org and download.openmpt.org run on the same server, using the same TLS configuration.

As I said, this type of connection error means that somebody between you and our server is tampering with the connection. Recently we found that this is typically due to antivirus software or internet server providers thinking we are serving malware, and thus blocking access to either individual OpenMPT downloads or the entire download.openmpt.org subdomain, like in your case.