MFC.down does the following:

            credits.decrement(length, block_time); //A
            if(needToSendCreditRequest()) //B
                    sendCreditRequest(tuple.getVal1(), Math.min(max_credits)

A blocks forever even if the MFC.max_block_time is configured:

<MFC max_credits="200k" min_threshold="0.20" max_block_time="1"/>

This happens at the same time on the whole cluster. B never gets invoked, so both wake up conditions( credits received or timeout) for credits.decrement are never satisfied resulting in the whole cluster to freeze.