While performing a rolling upgrade of an Availability Group from SQL Server 2016 CU1 to 2016 SP1 we noticed an issue. Immediately after we updated the secondary replica replication stopped working for that node and the SQL logs indicated there was an issue with the Database Mirroring login:
Database Mirroring login attempt by user 'DOMAIN\SERVICEACCOUNT.' failed with error: 'Connection handshake failed. The handshake verification failed. State 36.'. [CLIENT: 10.1.2.3]
What causes this issue and how can we resolve it?