Try a test without the NAS involved. They can sometimes have firmware bugs.
Hmmmm, but it will confirm nothing as I see it, because I don't have external drive that will fit all of my media library (not to mention that copy 1.5TB will take a while...), and if I copy some music on my MacMini SSD (about 50-80GB at most, as my SSD is only 128GB) and run auto-import, then if it's crashes it might mean that in this 50-80GB is a trigger file or it's some other issue, if JRiver don't crash then it might mean that it's NAS issue or trigger file is not in this 50-80GB of music..... so what exactly you propose me to do? ?
BTW, I'm using Synology DS414j NAS with latest DSM firmware...
Try without Splashtop.
OK, will try it today, but a bit later, because it quite a hassle, involves moving table near the rack, putting monitor on it and connecting monitor and keyboard + mouse to MacMini....