This is not really rtorrent-Pro issue, it's more QNAP itself. But more its all Linux embedded devices issue which are loading rootfs system into a small RAM disk where your non-existing download path was created and as soon it was out of free space.
Well once adding a torrent you have an option to use any disk volume mounted to it's corresponding share path. But instead that you have typed non-existing path. So rtorrent-Pro automatically created this path on rootfs which is running on small RAM disk and soon after torrent download starts this rootfs disk was out of free space.
Result was simple. Your OS was unable even to store a pid for running process and store system session logs. This is an limitation of all Linux-embedded system.