Do you have all Windows updates installed? I don't quire remember if a bare Windows 7 install supports all required HTTPS features or if an update is required (needless to say all security updates should absolutely be installed).

You could also go to the "Internet Options" settings dialog in Internet Explorer, go to the "Advanced" tab and double-check that the "Use TLS 1.2" checkbox is checked. If it's not checked, that might explain why OpenMPT cannot create a secure channel.

I have everything updated. That "Use TLS 1.2" thing doesn't work. I mean OpenMPT remains the same.

Is Internet Explorer able to access https://openmpt.org/ on that machine?

nope

There's probably little we can do then (there might still be some update missing - it works just fine here on my Windows 7 virtual machine), but maybe you can provide the error message you get from Internet Explorer, that might give a better idea what is actually going wrong.

If I'm not mistaken, not long ago I entered the page from Internet Explorer and it worked correctly.

Hmm, maybe this is a byproduct of Let's Encrypt's new root certificate, which is probably not trusted by older systems (and thus OpenMPT on those systems). Installing the ISRG Root X1 root certificate would probably make things work again in your IE installlation. But before you do that, knowing the exact error message from IE would be helpful because that could quickly explain if it's indeed the missing trust for the new root certificate or something different.

I also cannot reproduce on my fully updated Windows 7 setup.

In order to provide any further assistance we really need the error message(s) you get with Internet Explorer when trying to access https://openmpt.org/ .

Note that if you haven't used this Windows 7 installation in a while, or if it's a new installation, it might look like Windows has all available updates installed while it really is missing a whole lot of updates. That's because without one specific update, Windows 7 is no longer able to talk to its update servers.

Unless you are absolutely, positively sure that you have this update installed, I recommend following a guide like this one that lists all the updates that need to install manually. Once those are installed, Windows Update should work again.

Closing as the requested feedback was not provided and we're unable to reproduce the issue.