Hello @noris ,
after re-installing Roon things improved significantly, thx for this hint
nevertheless - as pointed out 2 weeks ago, please refer to Roon stopping randomly - #108 by fschmeis and Export hangs with spinning circle but no error message - I still have crashes, even with a media library shrinked by 90% (appr. 35.000 tracks instead of 350.000), last crash last evening 10:40 pm, please refer to “download_2021-12-02_22-42-09.zip”
I cannot enjoy my library for two weeks now as my favorite songs are deactivated 
I spent many hours to scan logfiles, run disk tests, run mentests - no errors, everything fine with my QNAP to my opinion
how to preceed?
PLEASE HELP