I am having this problem as well. I tried reducing the concurrency, but it did not appear to help. My D2D is a 4220( was a 4112 on 2.2.17) and upgraded to 3.50. I had to rebuild everything and it is possible that I have a config problem. It did work without any problems prior. It is still in being installed and not in production. This is a simple filesystem backup to two drives. The event log on the d2d reports session closed on all the drives and changer at the same time. Host does not show any errors in syslog.
Tommy