Try this instead
What does exactly join=true? Is this: if there's no conversation creat it, if there is such, join it?
Yes, basically, when using begin-conversation in pages.xml your only option is to use join=true
without it, a page refresh tries to start a new conversation, and nested=true means a new nested conversation is started each time the page is rendered, even if it is coming from an event handler or as part of navigation.