tmeyer is talking about an award ticket paying with M&M miles – which is the topic of the current thread - you are talking about a paid RWSTAR ticket. Both of you are correct, that neither LH nor SQ has local traffic rights JNB-CPT vv (this was the reason why tmeyer suggested to book this leg on SA, as a combination of LH/SA is possible in one award if paying with M&M miles, at least if the trip starts in Germany).

On a RWSTAR ticket the flight must be operated by *A carrier. As there is no *A carrier operating a domestic flight JNB-CPT vv you need a separate ticket for that leg (if you like to fly). In may wording, your CPT-JNB example is a surface sector. Basic I see two options: Skip the stopover at JNB and go CPT-FRA on LH or stay with that what you have booked.