Just an idea.
is your JMS Connection Pool too small ? If yes:
Increase JMS Connection Pool (default is 20)
in jbossesb.sar\jbossesb-properties.xml from 20 (default) to 40:
If it works, this was the cause of your problem.
In our experiences with ESB we've already done that. In fact we had lots of 'Unresponsive EPR's in high load situations. We increased the JMS Connection Pool (actually to a really exagerated value - 1000) and still got some. Then we saw a 'connections' setting in the file you mention. our prolems were solved when we increased that value.
This problem, however, is of a different kind. Is happens only in this situation, i.e., calling serviceInvoker from the MDB (from a WebService is ok). I'me trying to get some more log. We've had a similar problem with a web bean - the bean transaction somehow locks the esb transaction.
I've changed the MBean to NOT_SUPPORTED transaction and the problem disapear, now i'm only worried about failed message not going back to the queue since there is no transaction.