Job Owner - is the account that the job is owned by a sysadmin?
Job Step - Run As...does the proxy account have the necessary permissions?
When the owner of the job is a sysadmin account - the job will be run in the context of the agent service account and the job step will be run in the context of the proxy account.
When the owner of the job is a non-sysadmin account - the job will be run in the context of the non-sysadmin account and the job step will be run in the context of the proxy account.
Then - you can determine where the error is occurring and why. The job that is failing is most likely failing because the owner of the job (proxy account) does not have the necessary rights/permissions to execute the SSIS package.