The convertNamespaceFromWikitext.php script moves the existing wikitext talk page to a /Archive_NN subpage. This is a bad idea if the namespace is not in $wgNamespacesWithSubpages, since the archive page will still be a Flow board.
This happened in officewiki's Archive_talk namespace.
Most talk namespaces allow subpages. Probably the conversion script in a pre-flight check should warn and exit if the namespace does not.