There seems to be a bug in \components\com_sppagebuilder\addons\testimonial_carousel\site.php on line 441.

$css .= 'width:' . ((20 * $carousel_item->client_rating) - 2) . '%';

Unsupported operand types: string * int

I had to put an (int) before $carousel_item->client_rating. But of course this change will be removed after the next update.