Catch Everest Pro internal server error HTTP500 Appearance Customizer

Hello, when choosing Appearance, Customizer of when choosing Appearance, Header or Appearance, Background, Ia am getting this internal server error. This error only occurs with the Catch Everest Pro Theme and not with the standard Twenty Fifteen, Sixteen or Seventeen Theme. If I choose the Appearance Menu and then Theme Options, I will get the Theme Options Panel which says that it will be retired on future versions. It recommends me to use Customizer instead. The link to 'Customizer' also results in the error 500.