Login dialog disappears on MacOS

“No worries,”, I told my gf, “I’ll just download Nextcloud and login and all your files will be restored from my server, just like with iCloud”

Except, that’s not what happened and now she’s panicking and I’m a deceiver. Great.

“New Account” is a bit weird menu choice, isn’t it? As it’s NOT a new account, it’s an old account. Anyway, that was the only option so. I entered my server URL and clicked Next, then the login dialog appears for 0.1 second and then it’s gone. The only thing I see is a thin blue line on top of the dialog. See picture.
(I don’t know why the text is in Swedish, “Anslutningsguiden” ???. I sure didn’t select language. Yes the OS is in Swedish but that’s no excuse. Don’t make it Swedish unless I specifically say so!)