Symptoms
Assume that you are trying to upgrade a SQL Server 2016 mirrored instance to SQL Server 2017 by performing a rolling upgrade. After you complete upgrading, you may notice that the mirroring session is not able to synchronize. This may prevent you from completing the rolling upgrade. Additionally, an error message that resembles the following is logged in the SQL Server error log.
Error: 1479, Severity: 16, State: 1.
The mirroring connection to "<server network address>" has timed out for database "<database name>" after 60 seconds without a response. Check the service and network connections.
Resolution
This issue is fixed in the following cumulative update for SQL Server:
Cumulative Update 3 for SQL Server 2017
