I have given the system a couple days and tried again today.

I was successfully logged in with Twitter on the first attempt after enabling the twitter login, but cannot ever since. It only worked once (after an upgrade at my host service, therefore with a new IP address).
The re-directing does not work anymore. It just worked once.

The same way, trying to log with Google + redirects to the site after asking for permissions but without logging in. It never worked.