As of August 29th, SpamSieve was behaving normally.
Since then, it no longer filters out any spam (I use Mail, currently). Additionally, I cannot bring up most SpamSieve windows: it will bring up the log in Console, the SpamSieve Help window, and the SpamSieve Manual. Most other requests to bring up windows (Show Corpus, Preferences…, About SpamSieve, Edit Log, etc.) cause a brief blinking of the menu item in question – but nothing happens.
One interesting exception is Add Rule, which brings up a dialog box, all of whose controls are labeled with what I would guess are the control names, rather than with the English captions I’d generally expect (see attached png).
Following recommended procedure, I have not reset my corpus, deleted my preferences, or re-installed SpamSieve. The log is of no help: it last shows a “predicted good” message on August 29th that was, in fact, good.