Also, are you sure your session is created? If you haven't configured any session data, FuelPHP will default to cookie based sessions. And 1.5.1. had an issue with those, which was fixed yesterday afternoon. So if you installed before then, update your fuel/core.
Nope. After updating the core files the problem stays the same. (Without the code-editing I get redirected until my browser gives up, with it no $current_user.)
To be true: I have no idea if my session is created. How would I check this? I have not configured any session data, as I have no clue what would be the correct settings.
If you have not configured anything, Fuel will use cookie based sessions, which do not require any server side storage.
Make sure you are on the current 1.5.1. If you're not using a git clone, download https://github.com/fuel/core/archive/1.5/master.zip and install that in fuel/core. That way we can make sure you're not using the initial release which had a session bug that sneaked through the release validation process.
The problem still endures. (And is rather annoying.) Any clue what sessions might be wrong? Or missing? I used the standard settings. You can look for yourselfs: Here.