Can't upgrade back-end database that uses a SQL Server always on availability group to Cumulative Update 8 (CU8) or apply CU8 to Front End or Edge servers in Skype for Business Server 2015

Symptoms

In a Microsoft Skype for Business Server 2015 environment, when you try to upgrade the back-end database that uses a SQL Server always on availability group to the Skype for Business Server 2015 CU8 (January 2019 Cumulative Update) or apply CU8 to Front End or Edge servers, you may receive the following error message:

Install-CsDatabase : Command execution failed: The network path was not found.

Resolution

To fix this issue, install the May 2019 cumulative update 6.0.9319.548 for Skype for Business Server 2015, core components.

Workaround

For Edge and Front End servers, make sure that remote registry service isn't disabled. If it's disabled, enable it, set it to automatic, and apply the updates.

For the back-end database that uses a SQL Server always on availability group, take the following steps:

• Step 1: From SQL management studio, verify which SQL Server is the primary one for the availability group and note it.

• Step 2: In the SFB topology builder, expand Shared Components, right-click SQL Server Stores, right-click the SQL always on availability group used by the pool that you want to update, and then edit properties, change the "SQL Server FQDN:" from the <listener FQDN> to the  <SQL Server FQDN> which is currently primary as per step 1.

• Step 3: Publish the topology.

• Step 4: Run the Invoke-CsManagementStoreRepliction cmdlet to replicate CMS.

• Step 5: Verify that CMS replication is complete by using the Get-CsManagementStoreReplicationStatus cmdlet.

• Step 6: Run the install-csdatabase -update -ConfiguredDatabases -SqlServerFqdn <listener FQDN> -Verbose PowerShell command. 
  
  Notes: 

1. Listener FQDN is still used in the cmdlet.

2. If topology is being upgraded from Skype for Business Server 2015 Cumulative Update 7 (July 2018 Cumulative Update) to Cumulative Update 8 (January 2019 Cumulative Update), you can skip applying the changes that are made to the SQL Server databases. Therefore, you don't have to run the Install-CsDatabase cmdlet for this particular scenario.

• After the database is updated, follow step 2 and change the "SQL Server FQDN:" from the <SQL Server FQDN> to the <listener FQDN>. Then, publish the topology and start the CMS replication as per step 4.