I had a similar problem with some DM542 drivers and a parallel BOB after a little research it was suggested by a Mod on Machsupport forum that there seems to be a problem sometimes when using opto isolated Bob and opto isolated Drivers tends to be cheaper boards and may be a timing issue, but no one can find a concrete reason they should be affecting each other this way. Swapping opto Bob to non DM542 set up worked absolutely fine and mopving non opto bob across worked with DM542's so its apparent a problem exhists reported many times on machsupport forums but as yet no explanation as to why?
May well be nothing to do with OPTO isolation and its a red herring but coincidentally it seems to be a common denominator in many cases and cured by replacing offending BOB, and then offending Bob's work with non opto isolated drivers. would love to know the answer but suspect a timing problem occurs and the offending cards are just poorly designed and freeze.
I am trying a DDCM6v5.0 tomorrow for someone with couple of different different drivers so will let you know already tested with DM542 and no Mars bar.