Today we deployed one cube from Production to Development. On processing it we received error:
OLE DB error: OLE DB or ODBC error: A network-related or instance-specific error has occurred while establishing a connection to SQL Server. Server is not found or not accessible. Check if instance name is correct and if SQL Server is configured to allow remote connections. For more information see SQL Server Books Online.; 08001; Client unable to establish connection; 08001; Encryption not supported on the client.; 08001
Even though we had never changed the provider (the default setup gave us SQL native client), based on these 2 blogs (blog1, blog2) we changed it to OLE DB. This simple change allowed the cube to process successfully again.