Typing https://hostname in the login page doesn't work, even though this is a common way to specify the chat URL (e.g. in the Rocket.Chat+ client) -> remove scheme from what the user typed.
Actually, the proper way is probably do it like Rocket.Chat+ does: asking the user for a URL, not for a hostname. This allows setups like the one we initially had at KDAB where the chat URL also had a path, e.g. https://server.kdab.com/chat/. If this was still the case, I guess I wouldn't have been able to log in with the current ruqola?
Solved as can be seen here (https://cgit.kde.org/ruqola.git/commit/?id=8eda26184e742f03bf96a4691092a530e59b167f)