The group said Sonn stepped down on Monday in order to focus her attention on her position as chair of the supervisory board of Steinhoff International.
The group said Sonn contributed in steadying the company during a difficult period that was sparked by its parent company Steinhoff International's accounting irregularities last year. It said Philip Dieperink had been appointed to take over from Sonn.
STAR also appointed Mark Harris as an independent non-executive director, and he will also serve on the board’s remuneration committee.
It said Harris brought with him a broad range of business competencies, including strategy, mergers and acquisitions, innovation, technology and digitisation, crucial in the modern digital revolution.