[NTG-context] Transparency in PNGs becomes black in PDF/X

I think this is the same problem that I had in Bassenge: The transparent part of PNGs like the attached one becomes black if I try to make an PDF/X. I hope that is solvable... \setupbackend[format=PDF/X-1a:2001] \starttext \externalfigure[kinder/esel_kl] \stoptext Best, Hraban