django-registration activates user but shows error

Question

I'm using django-registration which somehow stopped working the way it should work. I'm completely lost on this and have no idea where to start looking for the error.

So what happens is, that whenever a user clicks on the activation link, he gets the activation.html template. Which should only be shown when there was an error.

But actually there is no error. The user gets activated and can log in.

Any thoughts on this will be greatly appreciated.