I think I’m good now. As I wrote earlier, I reinstalled and restored my backup overnight. The backup was made with the temporary network share library so I had to turn on local folders again and disable network share. After installing Build 850, I lost contact to core when I did this. Now I enabled local library and let it scan for files few minutes before disabling network share. I didn’t lose contact to core this time. Roon is still scanning my files as I write this but otherwise things seem to work normally now.
I have the newest QTS available from QNAP download center. 5.0.0.1837. Storage is on WD Red 4TB HDD in the NAS drive bay. QNAP model is TS-253D.