I don't know what it was, but I got rid of the problem by deleting the whole project and starting again. I assume it was just some sort of data corruption - it might have been brought about by copying-and-pasting into the html editor from a Word document, which seems to add a lot of weird code.
Anyway, all's well now.
General help
Error occurred, now unable to delete the offending topic
Denne diskusjonen har blitt stengt, og du kan derfor ikke svare i den lengre.