Ex2010 BE 2012 SP3 DAG backup comm failure 84f9 at 405GB of 570GB

Recently started getting backup failures per above on one of my three mailbox databases. 

At around the same time started moving some mailboxes off the 570GB DB to the second one to slim down the size. Some moves ended with warnings but the mailboxes moved. Other mailboxes moved without warning.

Also have been messing with circular logging to keep the log files down since the backups are not clearing them out. 

Have rebooted both the Ex 2010 mailbox servers and the backup server. I see SP4 is applicable so installing that now. 

Every 3 min am getting event 2137 log truncation failed for that database on the 2nd DAG member. 

Thanks for comments. 

TB