Hi guys, i am trying to use the Monitoring plugin for offline storage of OMEMO encrypted multiuser chat messages. The Monitoring plugin for Openfire does not save this type of messages, because their stanzas do not contain the body element. Is this a missing functionality in the Monitoring plugin (OMEMO with XEP-0313) or is it a concious decision? And if it is a conscious decision what are the reasons? Thanks.
Hm, usually OMEMO encrypted messages do not contain a body (or rather only a body with an informal message “I sent you an OMEMO encrypted message…”). Messages without a body are not stored by most servers, as they are not considered “valuable” for offline storage. This is because messages without a body are usually pretty short lived messages that an offline client has no interest in (eg. typing notifications etc).
However, in order to still have OMEMO messages stored by the server, most clients include a message processing hint which tells the server to store this message regardless of the missing body. I’d suspect, that either your client does not add that hint, or Openfire ignores it / does not know about it.
What client(s) are you using?
It is a custom client and the stanza includes the message processing hint : <store xmlns=‘urn:xmpp:hints’/>. So i guess it is a missing functionality from Monitoring plugin 
One of the developers said in chat this is likely not supported. You can file a ticket for that at https://github.com/igniterealtime/openfire-monitoring-plugin/issues
Thanks!