Hi,
64-bit DC (Distributed Clocks) is the clock synchronization scheme used in EtherCAT to synchronize time base of multiple EtherCAT slaves in the network something similar to IEEE1588. http://infosys.beckhoff.com/english.php?content=../content/1033/ethercatsystem/html/bt_ethercat_dc_intro.htm&id=5600
Anyway I am following up offline with Eddie. Will update here once we understand the issue details and find a solution.