PEP is broken, according to Conversations

I noticed that Conversation's "Server info" view reports support for PEP as "Broken". A look at the Conversations log shows this sequence:

07-11 20:59:47.656 D/conversations( 1863): AxolotlService (user@example.com): Own device 12345 not in PEP devicelist.
07-11 20:59:47.656 W/conversations( 1863): AxolotlService (user@example.com): Own device list empty, attempting to publish (try 1)
07-11 20:59:47.847 D/conversations( 1863): AxolotlService (user@example.com): Error received while publishing own device id<error xmlns="jabber:client" type="auth" code="403"><forbidden xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/></error>

We are using the default settings for PubSub in ejabberd.yml, with the exception of "ignore_pep_from_offline: false".

Any ideas why ejabberd might return 403 when trying to publish information?