Had the same issue myself on 6.1.9 after a reboot. Resolved by:
1. Deleting .dtach file from /detach_sess/.dtach on the docker (using command docker exec rm /detach_sess/.dtach.
2. Deleting rtorrent.lock file from dockers config folder (specifically /config/rtorrent/rtorrent_sess)
This seems to be reoccurring after every reboot of the container though (issue as described in quoted post, caused by .dtach and rtorrent.lock file being remade), surely this isn't expected behaviour and the files are supposed to delete itself on shutdown, anyone able to assist on a permanent solution?