multiple authentication backends configured and therefore must provide the `backend` argument or set the `backend` attribute on the user

I found the solution. call login() with backend argument like login(request, user, backend='django.contrib.auth.backends.ModelBackend')

here is full code:

def activate(request, uidb64, token, backend='django.contrib.auth.backends.ModelBackend'):
    try:
        uid = force_text(urlsafe_base64_decode(uidb64))
        user = User.objects.get(pk=uid)
    except (TypeError, ValueError, OverflowError, User.DoesNotExist):
        user = None

    if user is not None and account_activation_token.check_token(user, token):
        user.is_active = True
        user.profile.email_confirmed = True
        user.save()
        login(request, user, backend='django.contrib.auth.backends.ModelBackend')
        return redirect('home')
    else:
        return render(request, 'account_activation_invalid.html')