Possibly save 4 hours of your time: I notice some of my SSIS jobs are no longer working and the errors are different for each of them. But one error gave clue about the package variable issue.
When I run the package directly using environment variables, it works fine. But when I use a job with proxy/credential identity, it failed. So there is another hint this is related to permission issue.
What caught me off guard was these jobs used to work.
The jobs ftp files from/to the ftp server to a network location.
The variable in this case was a network path.
We change the environment variable to point to a local path and it works. This identifies that the issue has to do with the identity used to run the job does not have access to the network location defined by the variable.