Hi Basti,
thanks, I found the SO post myself. I had not set up the directives properly, so thought the fix didn't work. It does now. I also think they described a different problem, as in my case no cookies were sent, headers were fairly small and two requests with pretty much identical headers sent/received had different results (only 502ed).
I'm very interested in learning what exactly may have caused the message in my case, so as to know the boundaries in which my requests will work. I now set the buffer_size to 8K and I don't see failures with Content-Lengths that go far beyond that (though maybe compression has to be considered?).
Best regards,
Ruben