I reset the list and toggled the autoDE flag a few times. It did ask to prospect Cobalt and Adamantite, but not Saronite. NB: I did not clear the old SavedVariables or addon directories when 'installing' version 5.6.
It is already fixed. I just logged in again, pulled some Saronite Ore from the mail and the Prospect dialog appeared as normal. So it had something to do with the very first time 5.6 is loaded (against a 5.4 database).