Dear Sirs,
The solution was simple. Due to the fact that we were logged-in as Admin, we could not use the register function. This for sure sounds stupid (that is our own judgement too 😉 but we wanted to share this information here anyway so others can benefit from our experience.
Thus this function works well too and we here would like to give a big compliment for a great theme (KLEO).
Thus this issue is resolved.
Kindest regards,
Gerard.