[NTG-context] Strange behavior concerning pdf-output of externalfigures

Dear list, I'm getting unexplainable artefacts in the pdf-output. My MWE is: ------------------ \starttext \externalfigure [input.png] [width=100mm] \stoptext ------------------ I append the output "MWE.pdf" and the input-figure "input.png" for comparison. I'm experiencing the artefacts with some figures which are all several years old. Until now it never happened in conTeXt programs. Because I installed a new OS (LinuxMint) on my workstation at the same time I tried 3 pdfviewers (okular, evince, xpdf) all with the same result. I'm using LuaMetaTeX, Version 2.00.0, ConTeXt ver: 2019.04.13 17:25 MKIV beta fmt: 2019.4.15 Please, do you have any idea what could be the reason? Best wishes, Rudolf