When an archived message does not contain a stanza, but does contain body data, the monitoring plugin attempts to reconstruct the stanza. A string formatter is used here, but uses invalid parameters. This results in a stanza that contains placeholders, rather than actual data.
Fixed in monitoring plugin version 1.6.0.
The bug is still present in case of cluster configuration.
When a junior member submits a message to the senior member, as event of type "chatMessageReceived", the ConversationEvent run method passes an empty string to the conversationManager.processMessage method.
Regarding Momone's comment about the bug in a cluster, I found this code in ArchiveInterceptor.java: if the message comes from a senior member it gets processed and stored correctly, but if it is a junior member it gets sent to a queue with no stanza and hence is not saved correctly.
Maybe this is the cause?