Will there be a legal problem for Charles ascending to the throne?

None at all dear girl. The exclusion only relates to the current situation. Neither Charles or Camilla are currently RC, so no problem. I'll wager Prince Charles would be quite pleased that you take an interest in His affairs. It would be rather embarring if He had overlooked some simple impediment to His succession and through wonderful questions such as yours, these little things may well be discovered in the nick of time.