If you would like to monitor SQL Server with the SQL Server Browser service disabled (for security or other reasons), you must configure your database to use a static port assignment. If SQL Server is configured with static port allocation, an Instance Name value is not required for the SQL Server tablespace monitor to work.
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If you configure the database engine to use dynamic port allocation, ensure that SQL browser is runnning as it provides the connection information about the database engine to the the client. If the SQL browser service is not running and you have restarted SQL Server and port 1433 is being used by other applications, a different port number will be allocated to the database engine.
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If the client has been configured to use port 1433 to connect and the database engine is using a different port number, the client will not be able to get the connection properties to the database engine from SQL browser if SQL browser is not running.
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Additional details on SQL Server static vs. dynamic port allocation are available in this Microsoft KB article.