Problem signing in to server - Connection failure

[kiwichick888]

Server version: 10.11.6
Installed on: Windows 11 Pro 24H2

I have an issue that started a couple of weeks ago where someone tries to sign in, either on a new device or when changing user, but receives an error message: "Connection failure. We're unable to connect to the selected server right now. Please ensure it is running and try again". The server is definitely running because trying another user works, as does signing in using Quick Connect. The logs show an error for each time the connection failure happens: Jellyfin.Api.Middleware.ExceptionMiddleware: Error processing request. URL "POST" "/Users/authenticatebyname". Can anyone explain what's going on and how I can fix it?