I have a five-server farm -- working perfectly -- and I installed Office Web
Apps on each server, waiting until each was finished before running the SP
Config Wizard as Microsoft indicates in
technet.microsoft.com/.../ff431687.aspx

The Configuration Wizard ran until step 2, and then barfed on *every* server
with the following error (partial stack trace included just in case it
helps):

Failed to initiate the upgrade sequence.
An exception of type System.Data.SqlClient.SqlException was thrown.
Additional exception information: A network-related or instance-specific
error occurred while establishing a connection to SQL Server. The server was
not found or was not accessible. Verify that the instance name is correct
and that SQL Server is configured to allow remote connections. (provider:
SQL Network Interfaces, error: 25 - Connection string is not valid)
System.Data.SqlClient.SqlException: A network-related or instance-specific
error occurred while establishing a connection to SQL Server. The server was
not found or was not accessible. Verify that the instance name is correct
and that SQL Server is configured to allow remote connections. (provider:
SQL Network Interfaces, error: 25 - Connection string is not valid)
at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException
exception, Boolean breakConnection)
at
System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject
stateObj)
at System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo,
SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64
timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean
integratedSecurity, SqlConnection owningObject)

Anyone have a clue here? The SQL Server is definitely available, because I
can still get Central Admin and the main site collection to run. No amount
of Googling seems to turn up anything.

In case it matters, we're using a named instance on an uncommon port (though
the DB cluster does listen on the default port and will let you connect to
databases that way). We had to create an alias on the two app servers so
Forefront Identity Manager could find the sync database, which means that
any way you slice it, this thing should be able to connect.