I've just started experiencing this yesterday. I thought it might be because I upgraded 6.9.1 to 6.12.2 but not entirely sure how that could happen. Essentially, peers get connected for about 3 seconds, and then disappear again. No logs at all.
EDIT: I'd remembered that after updating Unraid one of the downloads finished, and there was an error in moving it to the completed folder. On a hunch, I decided to delete all the Downloads/* folders (completed, incomplete etc etc) and stop/start deluge. Everything now working. Highly recommend doing this. Bit odd that there were no errors or anything coming through in the logs?