Script level upgrade for database 'master' failed because upgrade step 'sqlagent100_msdb_upgrade.sql' encountered error 598, state 1, severity 25. This is a serious error condition which might interfere with regular operation and the database will be taken offline. If the error happened during upgrade of the 'master' database, it will prevent the entire SQL Server instance from starting. Examine the previous errorlog entries for errors, take the appropriate corrective actions and re-start the database so that the script upgrade steps run to completion

I encountered the below errors in SQL Error Log after completion of service pack / cumulative update on SQL Server 2008. Below are my work around to fix the issue

Error: 3417, Severity: 21, State: 3.

Cannot recover the master database. SQL Server is unable to run. Restore master from a full backup, repair it, or rebuild it. For more information about how to rebuild the master database, see SQL Server Books Online.

Cannot create file "\MSSQL10.\MSSQL\Data\temp_MS_AgentSigningCertificate_database.ldf' because it already exists. Change the file path or the file name, and retry the operation.
SQL Trace was stopped due to server shutdown. Trace ID = '1'. This is an informational message only; no user action is required.


Error: 1802, Severity: 16, State: 1.

CREATE DATABASE failed. Some file names listed could not be created. Check related errors.

WorkAround

1. Firstly we need to verify the SQL Server has enough permissions on data and log folders.

2. Need to verify the below registry Key Default Data Path is >> HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10.\Setup\SQLDataRoot

If this key has invalid path we can update the path and restart the SQL Server will solve the issue, if it has valid path and it still showing the error then

We need to move the temp_MS_AgentSigningCertificate_database.ldf & mdf to some other location and try to bring the service up from the cluster manager. Hope it will resolve the issue.