If you use a port other than the default (1433) to connect to your Microsoft SQL Server P6 Professional database, you might have to perform additional configuration steps to allow users access to functions within P6 Professional that rely on the P6 Integration API (for example, Update Baseline). Not all users will experience access issues. See Configuring Client Computers for a Non-Default Microsoft SQL Server Port.
Once you have determined which users are affected, you can either globally specify the connection string in the P6 Professional database SETTINGS table or install Microsoft SQL Server 2005 Backward Compatibility Components on each client machine.
See Database Administrator Settings for a Non-Default Microsoft SQL Server Port if you choose to specify the connection string.