Are you running Wayland or Xorg? I've had some troubles with Nvidia and Wayland before, though not like this. Just a guess, since I'm not using KDE

It's the same thing in emails, if you use the web application. All links are redirect links over their servers.

Not to be pedantic, but it's not shift+; it's :, which may be somewhere else depending on your keyboard. For example on (german?) Quertz that's shift+.

I tried the AIO image as well, I would recommend against it. https://github.com/nextcloud/docker is a more manual setup, but it's also much more flexible. AIO forces you to have a domain name and HTTPS certificate etc, which might not be necessary for you.

As for the page you are seeing, this is the administration page afaik, the actual nextcloud interface is running on a different port (https 443 with AIO).

You can go to your profile (middle tab), then switch accounts via the button in the top right