I had experienced a similar issue and it was because the scheduler did not have permissions of the file. The scheduler is unable to read the contents basically the login credentials from the file.
As a work around what I did was I created a separate job altogether to capture the password in the form of a secure string and then ran the job with the service Id. In this manner, the service has required access of password.
And just because it is the Id which is responsible for executing the job, it always ran well.