I've managed to find a solution. Video clips have to be captured vis the Firewire output and still pictures have to be captured via the USB output. Then everything works. What a pity the USB output will not do both, as advertised in the brochure.
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Alice Saey's mesmerising animation for Dutch singer Mark Lotterman
iPad Pro 10.5in (2017) review
Comment faire une capture d’écran sur un Mac ?