Some more information: I've found the possible fix in an upstream bugtracker at https://dev.deluge-torrent.org/ticket/3285. The semantics of the 'encoding' parameter are described at https://docs.python.org/3/library/pickle.html#pickle.Unpickler. The main problem is probably finding out what encoding the deluge code requires (latin-1 may also work?).
- Bug#954165: deluged: fails to load old torrent.state file with ... Jan Seeger
- Bug#954165: Supplemental information Jan Seeger
