I keep getting this too and found that it would "fix" itself if I restarted deluge. I'm using systemd to manage the service so I configured the service to restart every 24hrs, that way I don't have to think about this problem
Have you enabled the logging on the daemon? See the support section for the how to but it will tell more I guess in debug mode
Check the Libtorrent release too, or try another one maybe something similar in both setup