As a total hack, I tried adding a new catalogue:
Catalogue name: Extras
Web address: http://repository.maemo.org/extras/
Distribution: fremantle-1.3
Components: free non-free

and disabled the old extras. The app manager finds all the files and updates I would expect, and no more error. But I have no idea what could go wrong if I try to install apps from here. Do any of you know whether it should be OK. My guess is that the 1.2 repo will be back soon