Cannot connect to instance in Safari (desktop)

I had a problem with playing *.mov videos in Firefox (macOS). Some posts I found suggested that it would work in Safari searching, I wanted to test in Safari (e.g., this), so I wanted to try that for troubleshooting.

But I can’t navigate to my Nextcloud instance in Safari, I just get a:

Safari Can’t Connect tot the Server
Safari can’t open the page [REDACTED] because Safari can’t connect to the server [REDACTED].

From the same computer, I can connect just fine using other browsers, and the Desktop client also works fine from that computer. What the heck is going on here?