No, the date format of the server has not changed as other processes that uses a date would also have failed.
Check the default language for the SQL Server login
that the job is running under. That can be different to the default language for the server entire.
Hmm, if you're reading from a text file, the chances are the date format at issue may actually be associated with the Windows
login that the job is running under. Checking that would be a good idea as well.