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Executed as user: UKDBT91DB05V\\SYSTEM. Microsoft (R) SQL Server Execute Package Utility Version 10.50.2500.0 for 64-bit Copyright (C) M
This is probably a problem with your ProtectionLevel setting and how you are configuring the job.
If ProtectionLevel is at the default "EncryptSensitiveWithUserKey" setting, then the development environment is encrypting things like your connection string database password using a key that depends on the user who is doing the development. After you deploy it to the production server and schedule a SQL Server Agent job, it will most likely run under a different user account, and then it will be unable to decrypt the database password. You will get this error.
One possibility would be to make sure the development user and the job execution user are the same user account, but this is not a good idea. Usually you want your production user accounts to be separate from your development user accounts.
Instead, you should set ProtectionLevel to "DontSaveSensitive". Then the password won't get saved with the SSIS package at all. You create a configuration file for the connection string, but the password won't get saved to the configuration file either. You will have to edit the configuration file manually if you want it to include a password. But the best way to do this is to configure the password when you schedule the job that executes the SSIS package. That keeps the password in a safe place, and it isn't floating around all over the place with the SSIS package.
Some useful links here and here.
Good luck!