Thanks @Eric
I didn’t have the time yet to try everything you asked, but I can answer a few of the questions.
Roon fails through simple dropouts, the music stops for a second or two and resumes. No error messages at all. That’s it.
I only have FLACs in use, all containing PCM-files, except for one album in DSD64. I’m playing a lot using radio lately, can’t say there are specific files/tracks/albums that jump out, dropouts occur totally at random.
Without upsampling only these DSD64-files (no upsampling to DSD128, played in a loop) are presented with dropouts. As I’ve stated before (first post in this thread) I experience only dropouts when playing DSD-files or when upsampling to (double or single) DSD. Upsampling to PCM (up to) 24/384 is no problem.
Thumb-drive: not tried yet.
I use a Roonserver, no core. I tried to attach the server directly to the QB9DSD, but I need to bridge the ethernet ports to achieve that - and that works only for a short while 
See this topic … @eric, could you help out?
Marc