Have the same issue, and it is random.  Back up to flash drive and it works fine for several clients but get the System Argument error on one.  Spent hours with tech support today, apparently is a known bug but no ETA on getting a fix.  Their suggestion was to back up to new folder on desktop then copy paste to flash.  Does not work.  Client has sch D import with 300 entries, and over 100 of them were manually edited for wash sales, so do not want to lose this data (and I have been asking Lacerte for over a decade to add this function, but it falls on deaf ears).  

Just played around while on hold with tech support, and found that if I renumber the client file from xxx to xxxV2 it backs up just fine!  But then renumbering back it gets the error again.  This allows me to work on this file for now, but if you use REP or once you submit the efile you cannot renumber back to the original client number and retain the ack files.  So renumbering is only a temporary solution.