Thank you for the log, but I see nothing relevant to the error you mention
Transmission is using root, as everything on LibreELEC, eg you when you ssh
ssh into LibreELEC, try to write something to the directory Transmission is pointing to, and tell me what happens
It works just fine, I can write, read, delete in the directory.
What's strange is that it starts the downloads normally, saves the partial files on the directory and then (sometimes at 30%.... 50%) I get that error. It seems as the HD was suddenly unmounted. Any guesses?