I have the same problem since yesterday.
After seeing a movie on Plex, rTorrent has crashed.
No process found.
ExecStop=/usr/bin/killall -w -s 2 /usr/bin/rtorrent (code=exited, status=1/FAILURE).
This problem arrives even with new user created for the occasion.
And i don't know why but my script to make backups of the server has failed this morning, but he worked fine yesterday's morning.
"/usr/bin/rtorrent" file is here.
"/home/$user/.sessions/rtorrent.lock" isn't here for my 6 different users.
no "rutorrent-error.log" file found.
.rtorrent.rc looks fine.
I don't know what to search now ...
EDIT : found the solution (it worked for me but i don't know why).
Type in Terminal :
At startup torrent isn't working again. Try to start it with :
systemctl restart rtorrent@$user.service
check if the status is running
systemctl status rtorrent@$user.service
Don't forget to replace $user by your username.
As i said, now, i don't understand why a manual starting of rtorrent (and after a reboot !) worked. I tested without reboot before and it don't worked... Maybe reboot is the key