Thanks for your suggestion. I did try to insert this code where I thought I might help, though must admit that I'm not sure what to do with the argument [frontpart].

In my projectfiles, admittedly rather a mess, there is nothing with the name frontpart. Anyway, with or without this or some other possibly more appropriate argument, nothing changed the behaviour of this, or rather a blockquote in this first chapter, not even surrounding the blockquote itself with \setupinterlinespace[small] ...  \setupinterlinespace[reset], as I wrote in my original post. 

But, while I was writing this reply it occurred to me that maybe there might be some local variant for \setupinterlinespace[small]  and, yes, I found \setuplocalinterlinespace[line=4ex] on the ConTeXt wiki and this does the trick. (with [line-3ex]).

So you did help me, though maybe not in the way you intended ;-)

Thanks again,

Best regards,

Robert