I added debugging to print the sid and session->id on the error. The sid was empty. The session->id was a long value (maybe base64).
View source of editing page or preview+edit page shows no "sid" input value. (I do see it when editing here in your ikiwiki site, but not on mine.)
+
+> Further info. In the "prefs" dialog, it does have hidden "sid" defined. Viewing HTML source also shows this is in the fb_hidden class and Generated by CGI::FormBuilder v3.0501. I tried multiple times and never see "sid" in the HTML source of editing a page. --[[JeremyReed]]