I am not sure, but it may be related to what this other person has found.
I am testing Teamcity 7.04 (build 21474).
In the Global Configuration, the "Server URL:" is configured with the value "https://teamcity.mydomain.com"
When I type into a browser "https://teamcity.mydomain.com" it gets redirected to "http://teamcity.mydomain.com/login.html"
Because it drops the "s", the page fails to load.
I observed that if I adjust the address to "http://teamcity.mydomain.com/login.html", I am then able to successfully log in. After that point, since my session is established, I can type "https://teamcity.mydomain.com" and everything works as expected.
I believe there is something broken with the initial redirect to the logon page. It isn't honoring the Server URL global setting.