It's probably worth changing the SQL Agent account temporarily to see if it will run under a different account; I would suggest changing it to the virtual account installed alongside SQL Server, which should just be 'NT SERVICE\SQLSERVERAGENT'.

If you do that and it works successfully then it is likely down to the configuration of your account. Just double check that within Windows it is assigned the run as a service permission.

If that does not work then I would try doing a repair of the installation.