In case there is a wrong configuration for the "Redirect login to URL" setting, it won't be possible anymore to login to Cyclos. In that case, if the configuration problem is within a network, it is possible to use a global administrator to login in global mode (using the <server-root>/global/login URL), then switch to the network and fix the configuration. If the configuration error is in global mode, you can use a special URL to prevent redirect: <server-root>/global/login!noRedirect=true . However, this flag only works in global mode, to prevent end-users from using it to bypass the redirect.
Users should never have username / password requested in a plain HTTP connection. Always use a secure (HTTPS) connection. Also, just having an iframe with the form on a secure page, where the iframe itself is displayed in a plain page would encrypt the traffic, but browsers won't show the page as secure. Users won't notice that page as secure, could refuse to provide credentials in such situation.