We have had this a few times, intermittently and usually a couple of hours into treatment.
Sometimes it seemed to be a batch problem with BiBags, as the seal onto the connections is by interference rather than o-rings. We also ended up changing b-probe seals, rinse chambers and fitting the bibag filter retrofit kits, with varying degrees of success. One machine was swapped out and the problem has never recurred on the new or old machines.